Pakistani police fired tear gas and gunshots could be heard as a growing crowd of protesters converged on parliament on Tuesday, throwing stones at security forces, an AFP reporter said.
The protesters are followers of Canadian-Pakistani cleric Tahir-ul Qadri, who led a two-day protest march into the capital Islamabad overnight to demand a peaceful "revolution" and the dissolution of parliament.
It was not clear who fired the gunshots. An AFP reporter saw police fire tear gas shells at the crowd. Protesters were holding sticks and had pelted stones at police around 500 yards from parliament.
The AFP reporter said protesters smashed the windowpanes of vehicles.
Qadri told his followers to camp out overnight in Islamabad, despite chilly temperatures and advance towards parliament.
He gave the elected government, whose five-year mandate ends in March, until 11:00am (0600 GMT) to dissolve parliament.
"After that, the people's assembly here will take their own decision," he said, shouting and gesticulating from behind a bullet-proof box.
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 elections and sow political chaos.
Qadri's followers dismantled a first barricade of shipping containers separating the initial 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, struggling with a weak economy, crippling energy crisis and Islamist violence.