A spokesman for Nawaz Sharif says the Pakistani opposition leader left his residence this morning in defiance of an order placing him under house arrest.

A convoy of vehicles left Sharif's villa in the eastern city of Lahore on Sunday, apparently headed for an anti-government rally where police were battling stone-throwing demonstrators.

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Sharif spokesman Asif Kirmani confirmed that the former prime minister was in the convoy.

Sharif is locked in a bitter power struggle with Pakistan's president that threatens to paralyze politics in the nuclear-armed country and dilute its focus on tackling economic woes as well as Taliban militants operating along the Afghan border.

Hundreds of police surrounded the former prime minister's residence in the eastern city of Lahore before dawn and detained him along with scores of his supporters.

Officers showed party officials an order placing Sharif and his politician brother Shahbaz under house arrest for three days, party spokesman Pervaiz Rasheed said.

Shahbaz and a host of other protest leaders went underground to dodge similar orders. Rao Iftikhar, a senior government official, said the list of those facing detention also included the head of Pakistan's main Islamist party and a leader of its activist lawyers.

Lawyers and opposition party supporters had planned to gather today near Lahore's main court complex before heading toward Islamabad to stage a mass sit-in front of Parliament, in defiance of a government ban.

To thwart them, authorities parked trucks across major roads on the edge of the city, and riot police took up positions outside the railway station and government buildings.

About 1,000 flag-waving demonstrators jumped police barricades to reach the courts. At one point, protesters pelted some of the hundreds of riot police ringing the area with rocks.

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An Associated Press reporter saw one officer led away with a head wound before his colleagues fired two rounds of tear gas, scattering the crowd.

Television images also showed police commandos wearing flak jackets and armed with assault rifles apparently searching for Shahbaz in Rawalpindi, a city just south of the capital.

Shahbaz, speaking to Geo television by phone, appealed to ordinary Pakistanis to come out onto the streets.

"(President Asif Ali) Zardari has put the nation into this deep crisis by breaking his promises," he said. "These fascist tactics cannot stop the masses who want justice."

Zardari and Sharif are under pressure from Washington to reach a settlement. The US fears the dispute is bogging down the government just a year after democratic elections ended the long domination of former military ruler and US ally Pervez Musharraf — and preventing Islamabad from being an effective ally in the fight against militants in Afghanistan.

Suspected militants attacked a transport terminal in northwestern Pakistan used to supply NATO troops in Afghanistan before dawn and torched dozens of containers and military vehicles, including Humvees, police said.

The latest political turmoil began last month when the Supreme Court disqualified the Sharif brothers from elected office, over convictions dating back to an earlier chapter in Pakistan's turbulent political history.

Zardari compounded the crisis by dismissing the Sharifs' administration in Punjab, Pakistan's biggest and richest province.

The brothers then threw their support behind plans by lawyers to stage an indefinite sit-in in the capital — a move officials say would bring the government to a standstill and present a target to terrorists.

Yesterday, after US Secretary of State Hillary Clinton spoke to both Zardari and Nawaz Sharif by telephone, the government announced it would file an appeal against the Supreme Court ruling in the coming days.

Sharif's party welcomed the move but said it wouldn't compromise on demands for a shake-up of the judiciary.

Zardari refuses to reinstate a group of judges fired by Musharraf.

Many observers suspect Zardari fears the judges could challenge a pact signed by Musharraf that quashed long-standing corruption charges against him and his wife, slain former leader Benazir Bhutto.

Skeptics suspect Sharif of hoping to force early elections, from which he and Islamist parties would likely profit.