In a three-and-a-half hour speech delivered from behind bulletproof glass, Tahir-ul Qadri urged his followers to remain steadfast, despite the winter cold, in their demands for key reforms and clean elections.
He welcomed a Supreme Court order to arrest the prime minister over alleged corruption but gave no indication how long he would prolong the protest outside parliament, which has brought the city's main commercial avenue to a standstill.
"Keep sitting, don't move. Be steadfast. Your destiny is closer. There will soon be a decision in your favour," Mr Qadri shouted in a third fiery speech since reaching Islamabad after a 38-hour "march" from the eastern city of Lahore.
"It is now or never."
Security officials estimated the crowd numbered at least 25,000, 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.
The rally comes as Pakistan struggles with a weak economy, a bloody Islamist insurgency, rising sectarian violence, a sinking rupee and as fledgling peace gains with India look in jeopardy following a spate of cross-border shootings.
A general election is due to be held by mid-May, but Mr Qadri wants parliament dissolved now and a caretaker government set up in consultation with the military and the judiciary, to implement key reforms first.
But his sudden - and apparently well-financed - emergence after years in Canada has been criticised as a ploy by the establishment, particularly the armed forces, to delay the elections and sow political chaos.
If it goes ahead as scheduled, the ballot will mark the first democratic transfer of power between two elected civilian governments in the history of Pakistan, where the military have staged three coups and ruled for decades.
Mr Qadri denies wanting to delay the poll, but is calling for a new, independent election commission and screening to ban corrupt candidates.
The cleric, who as a dual Canadian-Pakistani national is not eligible for office, called on opposition politician Imran Khan and other parties to join him.
"I invite Imran Khan to come and join us. He also wants change... I also invite other parties, those who are not siding us," he said.
Khan, a former cricketer who leads the Pakistan Movement for Justice party but has no seat in parliament, has called on President Asif Ali Zardari to resign and for the government to set a date for elections.
So far, other political parties have backed the government over Mr Qadri, favouring parliament to remain in place until the government's mandate expires in mid-March.
But Tuesday's order to arrest Prime Minister Raja Pervez Ashraf sparked rumours of a judicial-military conspiracy to force the government out.
Commentators say there is no imminent sign that the prime minister will be arrested and that he can remain in office until and unless he is convicted.
The Supreme Court has been at loggerheads for years with the government, and last June Ashraf's predecessor was thrown out of office after the court found him guilty of contempt.
While critics dismiss them as a hired crowd, many of Mr Qadri's supporters articulate real concerns about Pakistan's problems.
With the rally now into a third day in Islamabad, the crowd appears to have dug in for the long-haul, with supplies of food, water and mattresses and blankets to cushion the kilometre-stretch of tarmac where they are bedded down.
Police clashed briefly with stone throwers and protesters on Tuesday, shooting into the air and firing tear gas. Eight officers were injured.
Rally organisers accused police of opening fire and trying to arrest Mr Qadri.