ISLAMABAD (AFP) - Pakistan's top judge Tuesday ordered the arrest of the prime minister over graft allegations, threatening to worsen turmoil as thousands of protesters demanded the government step down.

Chief Justice Iftikhar Muhammad Chaudhry ordered officials to arrest 16 people accused of corruption in power generation projects in 2010, including Prime Minister Raja Pervez Ashraf.

The Supreme Court order came as protesters led by populist cleric Tahir-ul Qadri massed near parliament on the third day of a march, calling for the immediate dissolution of the government.

A general election is due to be held by mid-May. But Mr Qadri wants a caretaker government set up immediately, in consultation with the military and the judiciary, to implement key reforms before the polls are held.

His demands are seen by critics as a ploy by elements of the establishment, particularly the armed forces, to delay the elections and sow political chaos in the nuclear-armed state, which was ruled by the military for decades.

Security officials estimated the size of the Islamabad crowd at between 25,000 and 50,000, which would make it the largest political protest in the capital since the government led by the Pakistan People's Party (PPP) was elected in 2008.

Opposition politician Imran Khan called on President Asif Ali Zardari to resign immediately and for the government to announce a date for elections.

The Supreme Court order signed by the chief justice, who has been at loggerheads for years with Zardari's government, will heighten an already febrile political atmosphere.

Analysts said the ruling would not force Mr Ashraf out of office. But they warned that even if the timing was a coincidence, coming at the time of the protest it would fuel rumours about a judicial-military conspiracy.

The court order instructed officials to arrest "without any hesitation" those accused in the case, and for the chairman and officials from the National Accountability Bureau corruption watchdog to report to the court on Thursday.

Mr Qadri's supporters, digging in for the long haul with stocks of food and bedding, cheered and danced when told of the order against Ashraf.

"This is our first victory. We will stay here until all our demands are met," Mr Qadri's deputy Sadiq Qureshi told the crowd.

The main index of the Karachi Stock Exchange fell sharply on news of the court order, losing nearly three percent in a little over half an hour. Markets closed early and people went home in panic in parts of the city, which has a tradition of unrest.