ISLAMABAD (AFP) - A cleric leading thousands of protesters was set for a showdown with the Pakistani government on Tuesday after marching on Islamabad to demand a peaceful "revolution" and the dissolution of parliament.

Mr Tahir-ul Qadri, 61, delivered an ultimatum in an overnight address to supporters in the heavily fortified capital following a 38-hour march from the eastern city of Lahore that attracted tens of thousands of supporters en route.

Mr Qadri told his followers to camp out overnight, despite chilly temperatures of eight degrees Celsius, and to advance after daybreak towards parliament, which has been barricaded off with shipping containers.

"I advise the administration, government employees and security forces don't be afraid... By tomorrow the government will have been changed, so don't worry, I have come here to free you from this slavery," he said.

Mr Qadri gave the elected government - whose five-year mandate ends in March ahead of general elections most likely in May - until 11:00 am local time to dissolve parliament or face a "democratic revolution".

"After that, the people's assembly here will take their own decision," he said, shouting and gesticulating from behind a bullet-proof box.

The influential and moderate cleric, who runs an educational and religious organisation with networks all over the world, 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 and for reforms has been seen by critics as a ploy by elements of the establishment, particularly the military, to delay the elections and sow political chaos.

It remains to be seen how the government and security forces, deployed en masse throughout Islamabad, will respond to his ultimatum.

After Mr Qadri's speech, his followers started to dismantle a first barricade of shipping containers separating the venue of the protest from parliament and other sensitive buildings in the government and diplomatic enclave.

His supporters say his calls to end corruption and implement reforms could be the solution to endless problems in Pakistan, brought to the brink by a weak economy, crippling energy crisis and Islamist violence.

"He can bring change for people who have long been fighting for their rights, usurped by the feudal and elite class," said Ms Huma Nadeem, a 20-year-old college student, in Islamabad.

"They are like beasts who have clung to power. They want to swallow everything," she added.

Mobile phone networks - sometimes used by Taliban militants to trigger bombs remotely - have been suspended as part of draconian security measures that have shut down much of the centre of Islamabad.

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

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

Mr Qadri wants a 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 the 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.