A general election is due to be held by mid-May but 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.
Critics see his demands 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 to be 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 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.
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," Qadri's deputy Sadiq Qureshi told the crowd.
Ashraf took office last June when the Supreme Court threw out his predecessor Yousuf Raza Gilani and convicted him of contempt for refusing to ask Swiss authorities to reopen corruption cases against the president. People convicted of certain crimes cannot be members of parliament under the constitution.
But senior lawyer Salman Akram Raja said, "Raja Pervez Ashraf can remain prime minister even after his arrest. Ashraf is only facing allegations and if he is detained for some investigation … he remains prime minister."
Political analyst and retired general Talat Masood called the timing of the supreme court ruling "amazing".
"It came when Qadri is saying the judiciary is great, army is great. It is adding weight to the instability in the country … acting as a catalyst for the dissolution of this government," he told AFP.
This article appeared in the South China Morning Post print edition as: Top judge orders arrest of Pakistan PM for graft
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