Detained Pakistani opposition leader Benazir Bhutto called on Tuesday for military leader Pervez Musharraf to step down as president, isolating him in the run-up to a general election.
Britain stepped up international pressure on Musharraf, who imposed emergency rule on November 3, backing a 10-day Commonwealth ultimatum for him to end the emergency and quit as army chief.
Bhutto has long called for Musharraf to step down as army chief and become a civilian president, but it was the first time she had called for him to quit as president altogether.
She also said she could never serve as prime minister under him and her party might boycott general elections Musharraf has promised to hold by January 9.
It is time for him to go. He must quit as president, Bhutto, who has for months held power-sharing negotiations with Musharraf, told Reuters in a telephone interview.
She was speaking from the city of Lahore where she was placed under house arrest for a week, hours before a planned protest procession against emergency rule.
Musharraf set off a storm of criticism when he imposed the emergency, suspended the constitution, sacked judges, locked up lawyers, rounded up thousands of activists and curbed the media.
The crisis has raised fears about stability in the nuclear-armed U.S. ally and concern about its ability to focus on battling a growing Islamist militancy.
Bhutto's party said 1,500 activists had been detained to thwart the long march motorcade from Lahore to Islamabad.
Clusters of supporters turned out and police detained many of them but there was no major violence in Lahore.
Unidentified gunmen opened fire on two police stations in Karachi while Bhutto's supporters were protesting against her detention but no one was hurt. In Peshawar, police used teargas and batons to disperse protesters.
Bhutto said Musharraf appeared out of his depth and had lost all credibility.
I will not serve as prime minister as long as Musharraf is president, she said.
Negotiations between us have broken down over the massive use of police force ... There's no question now of getting this back on track because anyone who is associated with General Musharraf gets contaminated, she said.
A spokesman for Musharraf, who took power in a 1999 coup, declined to comment.
Nearly 20,000 police were out in Lahore and about 4,000 of them moved in overnight around the house where Bhutto is staying, laying out coils of barbed wire and barricades and blocking streets with sand-laden tucks.
Her residence is an official jail now, said a senior officer. A detention order was pasted on the gate.
Police detained several party leaders and dozens of supporters chanting Go Musharraf go but after several hours hardly any supporters remained. A party leader said he had led a convoy into the countryside but police denied that.
Lahore is Pakistan's political nerve centre and the capital of Punjab province, which is ruled by Musharraf supporters.
Musharraf's power base is a group of disparate politicians who are expected to suffer heavy losses in the general election.
The general has seen his popularity slide since he tried to sack the chief justice in March, sparking protests by outraged lawyers who also took their campaign on the road with processions to Lahore and other towns across the country.
Bhutto was dogged by accusations of corruption during her two terms as prime minister but her party is Pakistan's biggest and has the capacity to mobilize huge crowds. The fact it hasn't had led to suspicion she was in cahoots with Musharraf.
Musharraf has come under growing pressure from Western allies to set Pakistan back on a path to democracy. He has declined to say when the constitution would be restored and said the emergency would ensure a fair vote.
U.N. Secretary-General Ban Ki-moon and U.S. President George W. Bush both urged Musharraf on Monday to lift the emergency.
The Commonwealth gave him until November 22 to end emergency rule and quit the army or face suspension. Pakistan was expelled after Musharraf's 1999 coup but let back in 2004.
British Foreign Secretary David Miliband, asked if Britain backed the call, said: Absolutely, the Commonwealth position was one that the UK played an important part in creating.
U.S. Deputy Secretary of State John Negroponte, who last week warned against cutting aid to an indispensable security ally, is due in Pakistan later this week on a long-planned visit.
Musharraf has justified the emergency by saying a meddling judiciary was hampering the battle against militants. Diplomats say he wanted to stop the Supreme Court from ruling invalid his October 6 election by loyalist legislators.
Musharraf has said he would step down as army chief and become a civilian president as soon as the Supreme Court, where new judges seen as friendly to the government have been appointed, ruled on challenges to his election.
(Additional reporting by Kamran Haider in LAHORE, Imtiaz Shah in KARACHI and Kate Kelland in LONDON; Writing by Robert Birsel; Editing by Bill Tarrant)