Former Pakistani Prime Minister Nawaz Sharif was arrested and deported to Saudi Arabia on Monday within hours of arriving home from exile vowing to end the rule of President Pervez Musharraf.
Authorities imposed a major clampdown before he flew in from London, detaining many leaders, spokesmen and activists of Sharif's Pakistan Muslim League party, stopping supporters from travelling to the capital and sealing off Islamabad airport.
Some of Sharif's supporters clashed with police as they tried to make their way past police barricades to get to the airport to greet their leader. Five people were hurt in an exchange of fire but protesters later dispersed.
Sharif's supporters said they would fight the government in the courts and politically.
Army chief Musharraf is preparing to seek another term in a presidential election in the national and provincial assemblies some time between Sept. 15 and Oct. 15.
A general election is due around the end of the year.
"For all practical purposes there is now martial law in Pakistan and Pervez Musharraf is the chief martial law administrator," said Sharif party spokesman Siddiq Farooq.
"We are going to take this issue up with the Supreme Court as well as with the people of Pakistan. We are exploring all legal and political avenues," he said.
The Supreme Court said last month Sharif had the right to return and the government should not try to stop him.
Government spokesman were not available for comment.
Sharif was arrested after a melee in an airport lounge where he and his supporters were taken after a tense 90-minute standoff with authorities on board the aircraft he arrived on.
He was deported to Saudi Arabia about four hours after flying in. A Saudi source said he would be accepted back into exile and was expected in the Red Sea port of Jeddah at 1230 GMT.
Another former prime minister in exile, Benazir Bhutto, is also expected to try to come home soon.
But she is in talks with Musharraf on a power-sharing deal that the president, whose popularity has slumped since he tried to dismiss the Supreme Court chief in March, hopes will help him secure another term.
Sharif's return from seven years in exile, most recently in London, was always going to spark a confrontation with Musharraf, who ousted Sharif in 1999 and cast him into exile in Saudi Arabia the following year.
Musharraf sent Sharif to Saudi Arabia under what the government says was an agreement that he stay in exile for 10 years. In return, he avoided a life sentence on hijacking and corruption charges.
Sharif was dogged by accusations of corruption during his two terms as prime minister in the 1990s. An anti-corruption court last month reopened three cases against him at the request of the government.
Before his arrival, authorities had detained about 4,000 Sharif supporters and party leaders, as well as three leaders of an allied religious alliance, party officials said.
Police said 250 "troublemakers" had been picked up.
Five people were wounded in an exchange of fire when Sharif's supporters tried to force their way through police lines on a bridge on the road to the city of Peshawar, a witness said.
Police fired teargas and used batons to disperse about 700 Sharif supporters chanting "Go Musharraf go" about 3 km away from the airport.
Shortly before his arrest, Sharif told Reuters he was happy to be home.
"It's a great feeling. Up to here it's fine but beyond, through there, I don't know," he said in an airport lounge, pointing to the exit.
(Additional reporting by Kamran Haider and Zeeshan Haider)