Former Pakistani Prime Minister Nawaz Sharif was arrested and taken into detention on Monday about three hours after arriving home from exile vowing to challenge President Pervez Musharraf.
Police took Sharif from an airport lounge and onto a bus, a Reuters reporter in the lounge said. A government official said he was being arrested on money-laundering and corruption charges.
Sharif's British lawyer, Amjad Malik, said Sharif was taken into detention on a helicopter.
"People in Pakistan should be calm and composed and understand what has happened to their former prime minister," Malik said.
"Let's move to court, let's have faith in justice."
Sharif's return from seven years in exile, most recently in London, was always going to spark a confrontation with General Musharraf, the army chief who ousted Sharif in 1999 and cast him into exile the following year.
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 the airport lounge, pointing to the exit.
He only left his aircraft after a tense 90-minute standoff with authorities. Surrounded by supporters, he was driven to a terminal building and entered the VIP lounge.
Security forces sealed off Islamabad airport to stop Sharif's supporters approaching. Authorities also blocked the main road to the northwestern city of Peshawar.
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 Peshawar, a witness said.
Earlier, police fired teargas and used batons to disperse about 700 Sharif supporters and lawyers about three km (two miles) away from the airport.
The protesters, waving party flags and held up portraits of Sharif, threw stones at police and chanted "Go Musharraf go". Scores of supporters scuffled with police in Islamabad.
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 several leaders of his Pakistan Muslim League (Nawaz), including the chairman, as well as three leaders of an allied religious alliance, party officials said.
Police said 250 "troublemakers" had been picked up.
Sharif, 57, sent into exile in Saudi Arabia in 2000, has returned home despite a Saudi official's plea for him to stay away for the sake of stability.
Musharraf exiled Sharif 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.
The government says he is breaking his word at a time when Pakistan needs stability in the run-up to elections.
His return is a major challenge for Musharraf, who has lost
support since trying to dismiss the country's top judge in March.
Musharraf is preparing to seek another term in a presidential election in the national and provincial assemblies some time between September 15 and October 15.
A general election is due around the end of the year.
The Supreme Court said last month Sharif had the right to return and the government should not try to stop him.
Pakistan says the Saudi royal family and assassinated Lebanese leader Rafik al-Hariri had guaranteed the exile deal. Sharif said on Saturday he understood the deal had been to stay away for five years.
(Additional reporting by Zeeshan Haider)