Anti-government protesters chant while walking with soldiers from the Pakistan Rangers during the Revolution March towards prime ministers house in Islamabad September 1, 2014. Photo: Reuters
Pakistan's national television channel is back on air after security forces removed anti-government protesters from its headquarters in Islamabad.
Troops were sent in to regain control from demonstrators who had forced their way into the PTV offices.
Earlier, fresh clashes erupted between protesters and police in the capital.
Anti-government protesters use sticks to hit a car and a motorcycle as they protest during Revolution March towards the prime minister's house in Islamabad September 1, 2014. Photo: Reuters
Protesters loyal to opposition leader Imran Khan and cleric Tahirul Qadri want PM Nawaz Sharif to resign. He denies corruption and electoral fraud.
Both Khan and Qadri have urged calm and asked their supporters to co-operate with the army.
A number of policemen are reported to have been injured in Monday's violence.
Thousands of demonstrators - some wielding batons and throwing stones - moved on the main building housing Pakistan's federal bureaucracy and Prime Minister's House.
Riot police clash with supporters of Tahir ul-Qadri, Sufi cleric and leader of political party Pakistan Awami Tehreek (PAT), during the Revolution March in Islamabad August 31. Photo: Reuters
Riot police were forced to retreat from the main road in front of parliament, Constitution Avenue.
Protesters attacked vehicles and set fire to shipping containers placed on the street as roadblocks.
Crowds of angry young protesters, many wielding batons, met little resistance as they stormed the PTV building.
Private news channels showed live pictures of protesters shouting slogans and barging into recording studios and smashing equipment.
Shortly afterwards troops arrived and peacefully escorted the demonstrators out of the building before transmissions resumed.
On Sunday night protesters used trucks to smash through the outer fence of the parliament building, even though the building was guarded by troops, the BBC's M Ilyas Khan in Islamabad reports.
Demonstrators have been taking part in a sit-in in the centre of the capital for two weeks.
Protests had been peaceful until Saturday, when violence broke out.
Last year's elections marked Pakistan's first civilian transfer of power. Sharif won by a landslide and BBC correspondents say the vote was deemed generally to have been fair.