The ousted chief minister of Pakistan's biggest province called on Friday for support for a lawyers' protest movement which is set to challenge the rule of a year-old civilian government.

Political tension mounted in the nuclear-armed U.S. ally, with fresh clashes between police and stone-throwing protesters enraged by a Supreme Court ruling this week that barred former prime minister Nawaz Sharif and his brother, Shahbaz, from elected office.

Nawaz Sharif is the main rival of President Asif Ali Zardari, widower of former prime minister Benazir Bhutto, and a power struggle between them could spell months of instability in a country grappling with a sagging economy and Islamist violence.

Protests have been held across Punjab province, the Sharifs' power-base and Pakistan's richest and politically most important province, since Wednesday's court ruling.

In Islamabad, protesters on Friday threw stones at police who responded with baton charges and teargas. There were no reports of serious injuries.

Police said Sharif supporters damaged a monument to Benazir Bhutto in Rawalpindi at the site of of her murder in 2007.

Shahbaz Sharif, ousted as chief minister of Punjab province this week when the Supreme Court nullified his election last year, told anti-government lawyers his party was behind them.

"If you want to save Pakistan, then you have to take part in the long march," Sharif told the crowd, referring to a cross-country lawyers' protest convoy that is expected to end with a sit-in outside parliament in the middle of next month.

The Sharifs see the Supreme Court as a tool of Zardari and they have refused to recognise the legitimacy of a chief justice he regards as an appointee of Pervez Musharraf, the former army chief forced to quit as president last August.

Anti-government lawyers, backed by the Sharifs, are campaigning for the reinstatement of the country's former top judge, Iftikhar Chaudhry, who was dismissed by Musharraf in 2007.

Zardari fears if Chaudhry is reinstated, he could rule that Musharraf's re-election as president in 2007 was invalid and also try him for violating the constitution by declaring emergency rule in November that year.

Chaudhry could then nullify an amnesty that Musharraf granted Bhutto and Zardari to enable them to return to Pakistan without fear of prosecution for old charges of corruption.

EFFIGY BURNED

Several hundred Sharif supporters protested in the town of Multan, burning an effigy of Zardari while his supporters attacked the car of a senior Sharif party official, a witness said. The official was not hurt.

If the country descended into chaos, the military, which has ruled Pakistan for more than half of the country's 61 years of history since independence, could be forced to act.

Some politicians, including smaller allies of Zardari, have offered to mediate, but chances of reconciliation appear remote.

In Punjab, the balance of power lies with the Pakistan Muslim League Quaid-e-Azam (PML-Q), which was carved out of Sharif's party by Musharraf to back his rule.

PML-Q politicians are being wooed by both sides to form a government once governor's rule, or direct rule by Zardari's representative imposed on Wednesday, ends.

Thirty three members of the PML-Q met Nawaz Sharif in the Punjab capital Lahore and assured him of their full support.