Former Pakistani prime minister and opposition leader Nawaz Sharif has been placed under house arrest in the city of Lahore, his party officials said on Sunday, hours before he was due to address a protest rally.

Sharif has thrown his support behind a protest campaign by anti-government lawyers that threatens to bring turmoil to Pakistan as the government struggles to stem militancy and to revive a flagging economy.

"Detention orders have been issued for Mr Nawaz Sharif, Shahbaz Sharif and other senior party leaders," Sharif party official Naseer Ahmed Bhutta said outside Sharif's house.

Sharif's brother, Shahbaz, is also a politician and a senior member of his party.

Police in riot gear virtually sealed off Sharif's house with road blocks on all approaches.

Government officials were not available for comment.

Police have detained hundreds of lawyers and opposition activists since a crackdown was launched on Wednesday in a bid to stifle plans by lawyers and opposition parties for a cross-country "long march" protest.

Nevertheless, black-suited lawyers and flag-waving opposition activists launched their campaign in the south of the country on Thursday, aiming to head to Islamabad.

But authorities quickly broke up the procession and snuffed out protests elsewhere with detentions, bans on rallies and roadblocks.

Protest leaders say they are still determined to hold a sit-in outside parliament in Islamabad on Monday. A looming showdown has raised fears of bloodshed on the streets.

In what appeared to be a step toward reconciliation with the opposition, the Pakistani government said on Saturday it would seek a review of a Supreme Court ruling last month that barred Sharif and his brother from elected office.

"The federal government will file a review petition in the Supreme Court against the verdict of the Supreme Court," a spokesman for President Asif Ali Zardari said in a statement.

Sharif was infuriated by the Supreme Court ruling last month that barred him and Shahbaz from elected office. The Sharifs said Zardari was behind the court decision.

But the secretary general of Sharif's party, Iqbal Zafar Jhagra, dismissed the move as "eye-wash" aimed at diverting attention from the protest campaign.

"A review petition means nothing. The long march will continue," Jhagra told Reuters.

The protesters' main demand is the reinstatement of former Supreme Court chief justice Iftikhar Chaudhry, who was dismissed in 2007 by the then president and army chief Pervez Musharraf.

President Zardari, widower of former prime minister Benazir Bhutto, has refused to reinstate the judge, seeing him as a threat to his own position.

Sharif had been due to address a rally in the centre of the eastern city of Lahore, his power base, later on Sunday as part of the protest.

If political the crisis gets out of hand, the army could feel compelled to intervene, though most analysts say a military takeover is highly unlikely.

The United States is deeply worried that the political crisis is a major distraction to Pakistan's efforts to eliminate Taliban and al Qaeda enclaves on the Afghan border, vital to US plans to stabilise Afghanistan and defeat al Qaeda. - Reuters