Police in Islamabad have reportedly used tear gas against anti-corruption protesters on a march led by cleric Tahirul Qadri.
The clashes erupted after demonstrators threw stones at security forces outside the parliament, reports say.
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 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 spokesman told Reuters news agency the protesters were trying to prevent security forces from arresting the cleric.
Gunshots have also been reported during the clashes but it is not clear who fired them.
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.