Tens of thousands joined protests in Pakistan's capital on Sunday led by opposition figures Imran Khan and cleric Tahir ul-Qadri, demanding the "corrupt" prime minister step down.
"Nawaz Sharif should resign as soon as possible. We will keep pushing forward, even into his bedroom," said Ali Gandapur, revenue minister in the province Khan's party controls, surrounded by rifle-carrying bodyguards.
The protests have raised worries about Pakistan's stability barely a year after Sharif took power in a landslide election win that marked the first democratic transfer of power in the history of the nuclear-armed nation of 180 million people.
Sharif has been struggling to overcome daily power cuts, a sluggish economy and a Taliban insurgency. Anti-Western militant groups have been growing in strength, worrying Pakistan's allies.
Khan, a former international cricket star whose speech is still peppered with metaphors of the game, told supporters in a late-night on Saturday speech: "maybe the world cup final will be played tomorrow ... people will forget Tahrir Square".
The reference to the square, site of Egypt's most violent anti-government protests, followed a warning by Khan earlier on Saturday. He said his supporters wanted to march on parliament via the "Red Zone", an enclave home to most Western embassies, and he was struggling to hold them back.
Many of the young men gathering at Khan's protest said they were eager to push against heavily fortified lines of riot police to reach parliament or the prime minister's house if Khan commanded.
"I told my parents, if I am martyred, pray for me," said student Muhammed Qasim, 21, his goatee painted in the red and green colours of Khan's party.
Riot police wearing body armour and carrying tear gas stacked shipping containers on top of each other and covered them with oil to prevent people climbing them behind the stage where Khan was due to speak.
The country's information minister, Pervez Rashid, told local television station Geo the government would not permit protesters to overrun government offices or the Red Zone.
"If they go to Red Zone, will the world see it?" Rashid asked. "This is our internal issue, but if they go into Red Zone, the issue will be heard in capitals across the world via their embassies. And there will be alarm bells in the capitals, signalling that Pakistan has turned into a insecure country."
Khan and Qadri arrived in Islamabad on Friday and say they will stay in the streets until Sharif, whom they condemn as corrupt, resigns.
Sharif's relationship with the powerful military has been poor, leading some in his government to suggest elements in the military are directing the protests to weaken the government and discourage it from policies it disapproves of.
Those include the prosecution for treason of former army chief and president Pervez Musharraf, who overthrew Sharif in a coup in 1999, ushering in a decade of military rule.
Despite Pakistan's history of coups, few feel the military wants to oust Sharif. The Supreme Court issued a warning on Friday against "unconstitutional" action.
Some analysts say the unrest has erupted because the opposition senses Sharif is vulnerable because of his rocky relations with the military, dithering over peace talks with the Taliban and failure to fill key ministries more than a year after taking office.
Qadri and Khan arrived in Islamabad late on Friday after a two-day procession through Punjab province, heartland of support for Sharif.
Qadri's supporters have set up camp on the capital's main thoroughfare, Jinnah Avenue, forcing many businesses to close.
"We are giving 48 hours for the government to resign and dissolve the assemblies and present themselves before the law," Qadri said Saturday night.
"Otherwise the people will decide and I will not be responsible."