LAHORE - Former Pakistani prime minister and opposition leader Nawaz Sharif was placed under house arrest on Sunday hours before he was due to address a protest rally.

Former Pakistani prime minister Nawaz Sharif speaks during a news conference in Islamabad, in this August 25, 2008 file photo. [Agencies]

"A senior police officer is here and he informed Mr Sharif that he's been detained for three days," said party spokesman Pervez Rasheed. Police confirmed the order.

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

Police later fired tear gas outside the High Court in the city centre to disperse stone-throwing protesters, a witness said.

Hours after Sharif was detained, police placed top lawyer and protest organiser Aitzaz Ahsan under house arrest, his aide said.

Parliament Sit-in

Police have detained hundreds of lawyers and opposition activists Wednesday to prevent their cross-country "long march" protest that is due to climax with a sit-in outside parliament in Islamabad on Monday. The government has also asked troops to be ready, if needed to restore order.

Sharif had been due to address a rally in Lahore, his power base, on Sunday and then head to Islamabad along with other protest leaders such as Ahsan.

If the political 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 crisis is a 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.

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

But the secretary general of Sharif's party, Iqbal Zafar Jhagra, dismissed the move as "eye-wash" and said the protest would go on.

The Sharifs said Zardari was behind the ruling, which was based on old convictions the Sharifs say were politically motivated.