Islamabad  - Pakistan's top opposition leader Nawaz Sharif Sunday, in apparent defiance of his alleged house arrest, broke through multiple cordons, and was leading a massive anti-government rally that is heading towards capital Islamabad.

The development deepened political turmoil in the nuclear-armed country which is a key ally of the West in the international fight against terrorism.

Tens of thousands of Sharif's supporters thronged streets as he left his residence in a dark green armoured Land Cruiser in eastern city of Lahore, despite a government ban on public gathering and a massive deployment of law-enforcement personnel.

"It's a sea of people. It looks as though the entire Lahore city is gathered here. Young, elders, women, children - everyone is here," local journalist Younis Baath said.

The crowd chanted slogans against the government of President Asif Ali Zardari, including "Death to Zardari," "Zardari is a dog" and "Restore the judges." There were also calls for revolution and change.

"It's a rare moment of Pakistan's history. It's prelude to revolution," Sharif told Geo television in a phone interview as he led the rally.

The two-time prime minister and right-wing leader publicly supported a campaign spearheaded by defiant lawyers seeking reinstatement of judges, including former Supreme Court chief justice Iftikhar Chaudhry, who was removed by ex-president Pervez Musharraf in 2007.

Zardari, the widower of former slain premier Benazir Bhutto, promised to restore the judges after he took over last year, but refused to do so later on, apparently out of fear that independent-minded judge might revive corruption charges against him.

But the broken promises turned him into a highly unpopular political figure.

Sharif plans to lead a so-called long march from Lahore to Islamabad for a sit-in scheduled to begin on Monday until his demand is met.

"Our destination is Islamabad. We have left for Islamabad. The government should listen to the voice of people and accept our demands," said Sharif, who has been shown by some recent polls as the country's most popular leader.

Earlier, hundreds of policemen surrounded Sharif's residence and sealed all the access roads, Ahsan Iqbal, a spokesman of his Pakistan Muslim League-Nawaz party told Deutsche Presse-Agentur dpa.

"Come and join me. I am leaving the house. The time has come to march hand-in-hand," Sharif told a charged crowd before leaving as his party workers removed the road blocks. The police officers stood aside.

Some 15 kilometres from Sharif's residence, in the heart of Lahore, police using hundreds of canisters of tear gas and batons tried in vain to disperse his supporters who pelted security personnel with stones.

According to some media reports, several people were injured, including some policemen.

Law enforcers retreated following hours-long clashes and some policemen joined the crowd and chanted slogans in favour of Sharif, Geo reported.

The civilian administrator of the city, Sajjad Bhutta, refused to accept government orders while the country's deputy attorney general resigned from his post in protest against use of force against the protesters, the report said further.

Principal roads in nearly all main towns along Sharif's planned route to Islamabad were blocked using shipping containers, lorries and concrete barriers. The government also blocked text service on mobile phones and transmissions of some television news channel in capital.

More than 1,000 activists are already in police custody amid a countrywide crackdown on opposition groups to ward off the Islamabad rally.

TV footage showed Sharif's supporters using cranes to remove the hurdles and clear the route.

The political turmoil has raised concerns in Washington and other Western capitals which want Zardari and Sharif to end their feud and join efforts against Islamic extremism.

US and British officials have tried to mediate between the two traditional rivals, who briefly joined hands in early 2008 to defeat political allies of former military strongman Musharraf and remove him from the president's office.

Sharif differed on the issue of the judges and the rift between them widened after last months' court ruling that barred Sharif and his brother, Shahbaz Sharif, from elected office.

The verdict also dismissed Shahbaz Sharif's provincial government in Punjab, Lahore, a stronghold of the brothers.

The Zardari government announced some concessions for the opposition leader late Saturday following phone calls from US Secretary of State Hillary Rodham Clinton to Zardari and Sharif.

A presidential spokesman said the government would appeal the disqualification of Sharif brothers and pledged to resolve the issue of the reinstatement of judges sacked by Musharraf.

But the opposition groups rejected the offer saying only concrete actions would satisfy them, not merely announcements of good intentions.