Cricketer-turned-politician Imran Khan and populist cleric Tahir-ul-Qadri had on Tuesday led followers in a late-night march on the parliament building, the culmination of a week-long standoff with prime minister Nawaz Sharif.
Khan and Qadri say last year's general election that swept Sharif to power by a landslide was rigged, and they are demanding his resignation.
Qadri repeated his demand for Sharif to quit and install a "national government", and ordered his followers to stop lawmakers leaving a national assembly sitting called to debate the crisis.
His activists occupied the main entrances to parliament but MPs left the building by a back entrance without incident.
Elsewhere in Islamabad's high-security government "red zone", followers of Qadri's Pakistan Awami Tehreek movement blocked the entrance to an office complex housing numerous ministries.
Under the gaze of riot police, they said they would not allow any ministers or MPs to leave until Sharif quit.
The showdown has added to the sense of instability in a country struggling with a homegrown Taliban insurgency, a crippling power crisis and a sluggish economy.
Neither protest leader has shown any signs of backing down, despite repeated government offers of talks.
But on Wednesday the Supreme Court, which has played an influential role in Pakistani politics in recent years, ordered Khan and Qadri to appear on Thursday to explain themselves, a court official said.
The ruling came after petitions urging the court to restrain Khan and Qadri from "making illegal and unconstitutional demands", Kamran Murtaza, a senior lawyer, told AFP.
There had been fears the protesters' advance on parliament could trigger clashes, but riot police and other security forces looked on without intervening.
What's happeneing in 'Red Zone'
Supporters of Pakistan Tehreek-e-Insaf (PTI) and Pakistan Awami Tehreek (PAT) entered Islamabad's high security red zone on Tuesday night after clearing roadblocks placed in their way and on Wednesday morning had assembled in front of parliament house with Imran Khan promising that prime minister Nawaz Sharif would be resigning by Wednesday evening.
"If Nawaz Sharif does not resign, we will enter into the PM House," Khan said while addressing to protesters, outside the parliament.
The rallies led by Pakistan Awami Tehreek and Pakistan Tehreek-e-Insaf marched into the Red Zone and set up camp in front of the Parliament House, even as the government, opposition members and the army urging all stakeholders to end the impasse via talks.
"Promise me, if something were to happen to me, you will take revenge from Nawaz Sharif," Khan said before starting his march towards the Red Zone that houses important government buildings including the parliament house, prime minister house, president house, the supreme court besides embassies.
However, it is not clear how this objective will be achieved under the present circumstances, say observers.
The government has not resisted the marchers but has deputed army at sensitive installations and buildings in the Red Zone.
Police officials were instructed to let the marchers come into the red zone. Interior minister Chaudhry Nisar told the media on Wednesday morning that the government "Was doing everything it could to avoid a confrontation."
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Pakistani religious cleric Tahir-ul-Qadri flashes "V" sign to his supporters during a massive rally in front of The Parliament building in Islamabad. (AP Photo)
"The army has warned them from entering the parliament building or any state institution and this has effectively stopped them making any plans to do so," commented journalist Ansar Abbasi.
Abbasi said that the only way ahead for the protestors would be to create a situation that calls for desperate measures from the side of the government. "This can only mean resorting to violence," he added.
Sharif decides to meet Khan
Sharif has decided to meet Khan in an effort to end the anti-government protests in Islamabad seeking his ouster.
"It has been decided PM (Sharif) will meet Imran Khan for the sake of country," minister for railways Khawaja Saad Rafique, a close aide of Prime Minister Sharif, tweeted.
Rafique did not give the time for meeting.
Read | Islamabad on edge as Imran Khan, Qadri supporters reach Parliament
Pakistani opposition politician Imran Khan gestures during a protest march against the country's Pakistan Muslim League-Nawaz-led government in Islamabad. (AFP Photo)
Qadri says 'won't storm in PM house'
Qadri said late Tuesday that a session of Awami Parliament (People's Parliament) will be held in front of the Parliament House on Wednesday, Geo News reported.
In a statement in Islamabad, Qadri said his party will make stage in front of the Pakistan parliament and he would welcome Pakistan Tehreek-i-Insaf's chief Imran Khan at his party's stage.
"Whether or not Khan storm into PM House, we will not do so", Qadri said, adding the capture and destruction to government buildings is not part of their plan.
Stating PAT will not ask military for mediation or intervention, Qadri said prime minister Nawaz Sharif and chief minister Shahbaz Sharif should surrender before the law of the land.
Pak army back in the driving seat
Pakistan prime minister Nawaz Sharif has been assured by the country's military there will be no coup, but in return he must "share space with the army", according to a government source who was privy to recent talks between the two sides.
Last week, as tens of thousands of protesters advanced on the Pakistani capital to demand his resignation, Sharif dispatched two emissaries to consult with the army chief.
He wanted to know if the military was quietly engineering the twin protest movements by Khan and Tahir ul-Qadri, or if, perhaps, it was preparing to stage a coup.
According to a government insider with a first-hand account of the meeting, Sharif's envoys returned with good news and bad: there will be no coup but if he wants his government to survive, from now on it will have to share space with the army.
The army's media wing declined to comment on the meeting.
Thousands of protesters marched to parliament on Tuesday, using a crane and bolt cutters to force their way past barricades of shipping containers, as riot police and paramilitaries watched on after being told not to intervene.
Military spokesman General Asim Bajwa tweeted a reminder to protesters to respect government institutions and called for "meaningful dialogue" to resolve the crisis.
Even if, as seems likely, the Khan and Qadri protests eventually fizzle out due to a lack of overt support from the military, the prime minister will emerge weakened from the crisis in coup-prone Pakistan.
Sharif may have to be subservient to the generals on issues he wanted to handle himself - from the fight against the Taliban to relations with arch foe India and Pakistan's role in neighbouring, post-NATO Afghanistan.
"The biggest loser will be Nawaz, cut down to size both by puny political rivals and the powerful army," said a government minister who asked not to be named. "From this moment on, he'll always be looking over his shoulder."
A year ago, few would have predicted that Sharif would be in such trouble: back then, he had just swept to power for a third time in a milestone poll that marked nuclear-armed Pakistan's first transition from one elected government to another.
But in the months that followed, Sharif - who had crossed swords with the army in the past - moved to enhance the clout of the civilian government in a country that has been ruled by the military for more than half of its turbulent history.
He irked the generals by putting former military head Pervez Musharraf, who had ended Sharif's last stint as prime minister in a 1999 coup, on trial for treason.
Sharif also opposed a military offensive to crush Taliban insurgents, sided with a media group that had accused the military of shooting one of its journalists and sought reconciliation with India, the perceived threat that the army uses to justify its huge budget and national importance.
Thousands of supporters of anti-government cleric Tahir-ul-Qadri dance during a march in front of parliament building in Islamabad. (AP Photo) dismissed the proposal immediately.
India rapprochement at risk
Sources in Sharif's government said that, with civilian-military relations in such bad shape, Sharif suspected that the street protests to unseat him were being manipulated from behind the scenes by the army.
He also feared that, if the agitations turned violent, the army would exploit the situation to seize power for itself.
However, the two close aides who went to see army chief Raheel Sharif in the garrison town of Rawalpindi last Wednesday were told that the military had no intention of intervening.
"The military does not intend to carry out a coup but ... if the government wants to get through its many problems and the four remaining years of its term, it has to share space with the army," said the insider, summing up the message they were given.
"Sharing space" is a familiar euphemism for civilian governments focusing narrowly on domestic political affairs and leaving security and strategic policy to the army.
The fact that the military is back in the driving seat will make it harder for Sharif to deliver the rapprochement with India that he promised when he won the election last year.
Indian media speculated this week that Sharif had already been forced by the generals to scuttle peace talks.
New Delhi on Monday called off a meeting between foreign ministry officials of the two countries, which had been set to take place on Aug. 25, because Pakistan announced its intention to consult Kashmiri separatists ahead of the meeting.
The Pakistani army's predominance could also mean it could torpedo the government's relationship with Afghanistan, where a regional jostle for influence is expected to intensify after the withdrawal of most foreign forces at the end of this year.
Sharif paying the price
Few believed that the army would back Khan's bid for power even if it used him to put Sharif on the defensive.
"Even the army knows that Imran Khan may be a great pressure cooker in the kitchen, but you can't trust him to be the chef," said a former intelligence chief who declined to be named.
Sharif may now pay the price for miscalculating that the military may have been willing to let the one-time cricket hero topple him.
"Thinking that Imran could be a game-changer, Nawaz has conceded the maximum to the army," a Sharif aide said.
"From a czar-like prime minister, they (the army) have reduced him to a deputy commissioner-type character who will deal with the day-to-day running of the country while they take care of the important stuff like Afghanistan and India. This is not a small loss."
But Sharif's aides say a stint in jail under Musharraf, followed by exile from Pakistan and five years as leader of the opposition party, have made him realise that he needs to share power to survive.
"This is not the old Nawaz, the wild confrontationalist," said an adviser to the prime minister in Lahore, the capital of his Punjab province power base. "This is the new Nawaz who has learnt the hard way that politics is about living to fight another day."
US urges for peace
The United States has appealed to all sides in Pakistan to refrain from violence and resolve their differences through peaceful discussions in a way that strengthens democracy in the country.
"We are carefully monitoring the demonstrations in Islamabad. We urge all sides to refrain from violence, exercise restraint, and respect the rule of law," state department deputy spokesperson Marie Harf told reporters at her daily news conference.
"We believe that all parties here should work together to resolve their differences through peaceful dialogue in a way that strengthens Pakistan's democracy, and that's certainly the consistent message we have sent," she said in response to a question.
(With inputs from HT, AFP, PTI, Reuters, AP)