Police in Islamabad have fired warning shots in the air and used tear gas against anti-corruption protesters on a march led by cleric Tahirul Qadri.
The clashes broke out briefly after demonstrators threw stones at security forces outside parliament.
Mr Qadri, who is calling for electoral reform, left the city of Lahore on Sunday with thousands of supporters, and reached Islamabad late on Monday.
Authorities accuse him of trying to postpone elections due by May.
The Sunni cleric wants the military and judiciary to be involved in installing a caretaker government to oversee the forthcoming elections.
The government is due to disband in March, and elections must then be held within six weeks.
Mr Qadri's march has been otherwise peaceful so far, but live television coverage on Tuesday showed police firing shots into the air and using tear gas to push back protesters who were hurling stones at officers.
Mr Qadri's spokesman told Reuters news agency the demonstrators were trying to prevent security forces from arresting the cleric.
Reports of injuries have not yet been verified independently.
'Believer in democracy'
Earlier, authorities in the capital had said Mr Qadri and his supporters would not be allowed into the city centre. The government had also warned that militants could target the marchers.
An extra 15,000 police were deployed and many parts of the capital sealed off.
Addressing tens of thousands of supporters late on Monday night, Mr Qadri called for provincial assemblies to make way for a caretaker administration.
He wants measures put in place to prevent corrupt people or criminals from standing for elected office.
"Morally, your government and your assemblies have ended tonight," he said from behind bullet-proof glass on a stage erected on Jinnah Avenue, less than a mile from parliament.
"I will give [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."
By the time his procession reached Islamabad, an estimated 10,000 people had joined the slow-moving convoy of cars, buses and trucks - more crowds were waiting in Islamabad to greet the cleric.
Tahirul Qadri's flamboyant preaching style and expensive television campaigns have raised his profile in Pakistan in recent weeks.
But there has also been widespread speculation that he is backed by Pakistan's powerful military, and is being used to reassert the army's control over Pakistani politics.
Mr Qadri has rejected this allegation. "I have no link with military institutions," he told Reuters earlier. "I am one of the biggest staunch believers... of democracy in the whole world."
The preacher was a prominent supporter of former army chief Pervez Musharraf when he seized power in a coup in 1999, and served in the national assembly under him before moving to Canada in 2006, where he ran a charity.
In December, he returned to Pakistan and was able to mobilise tens of thousands of supporters at a rally in Lahore, but it remains unclear how much support he enjoys across the country.