A cleric calling for revolution in Pakistan gave the government an ultimatum Tuesday to dissolve parliament and make way for a caretaker administration, whipping up a mass protest rally in Islamabad.

Tahir-ul Qadri, 61, addressed crowds gathered on a main thoroughfare of the heavily fortified capital, following a 38-hour protest march from the eastern city of Lahore that attracted crowds of tens of thousands along the route.

Qadri told his followers to camp overnight, despite temperatures plummeting to 8 degrees Celsius (46 degrees Fahrenheit), and after daybreak to advance towards parliament which has been barricaded off by shipping containers.

"There will be justice tomorrow. You will have to stay tonight, sleep here, go outside parliament and tomorrow after 11 am (0600 GMT), it will be the inaugural address of the people's democratic revolution," Qadri said.

The influential and moderate cleric, who runs an educational and religious organisation with networks all over the world, only returned to Pakistan last month from years spent living in Canada, where he also has citizenship.

His demand for the military to have a say in a caretaker administration has been seen by critics as a ploy by elements of the establishment, particularly the military, to delay elections and sow political chaos.

But his supporters say they are drawn to his calls to end corruption and implement reforms in a country brought to the brink by a weak economy, crippling energy crisis and Islamist militant violence.

"I will give you (the government) a deadline until tomorrow to dissolve the federal parliament and provincial assemblies. After that, the people's assembly here will take their own decision," said Qadri, stabbing the air with his forefinger from behind a bullet-proof box erected on a podium.

He was chauffeur-driven in a sleek black car and showered with rose petals as he arrived in Islamabad, where thousands waved Pakistani flags.

Mobile phone networks have been suspended as part of draconian security measures that have shut down much of the centre of the capital to guard against what the government says is a threat of Taliban attacks.

Thousands of security personnel were deployed while paramilitary soldiers, police and private guards searched all those trying to access the venue.

The lack of communications made it impossible to verify the size of the crowd. Qadri claimed to have mobilised four million people, but local TV station Geo quoted various officials as putting the number between 25,000 to 40,000.

Two security officials told AFP that they estimated the turnout in Islamabad at 15,000 to 20,000. Officials had said 50,000 people turned out to greet Qadri earlier in the day along the route or to join his convoy.

Much of the significance of the rally will hinge on turnout. Observers suggest that a show of under 100,000 would be a wash-out for the cleric.

Qadri wants the caretaker government set up in consultation with the military and judiciary when parliament disbands in mid-March, and is calling for reform so that "honest people" can be elected at polls due by mid-May.

If held on schedule, the election will mark the first democratic transition of power between two civilian governments in Pakistan's 65-year history, which has been marked by bloodless coups and extensive periods of military rule.

Earlier in the day, an AFP reporter saw men, women and children piled onto the rooftops of buses, flashing victory signs in a five-kilometre convoy.

They left Lahore on Sunday and passed through towns and villages waving Pakistani flags and dancing to drumbeats.

"I'll stay there until real change comes or until Tahir-ul Qadri asks me to go back. If I have to stay 10 years, I'll stay there," said Hafez Aamir Chishti, a cleric from Lahore who joined the protest march on his motorbike.