Media playback is unsupported on your device Media caption Protesters cut cables and smashed broadcast equipment, as Shahzeb Jillani reports

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.

Protesters loyal to opposition leader Imran Khan and cleric Tahirul Qadri want PM Nawaz Sharif to resign. He denies corruption and electoral fraud.

Both Mr Khan and Mr Qadri have called for calm and asked their supporters to co-operate with the army.

Media playback is unsupported on your device Media caption As demonstrators stormed the PTV station, the presenter said there was ''no reason to panic''

Mr Sharif, who insists he will not quit, has met the country's powerful army chief, Gen Raheel Sharif, for talks on the crisis. The president has called a joint session of parliament for Tuesday.

Analysis: M Ilyas Khan, BBC News, Islamabad

The brief but alarming occupation of Pakistan Television underlines a deepening political crisis. The fact that anti-government activists got into a building that troops were apparently guarding has also prompted some to question the army's role.

Troops did move in swiftly to eject the protesters, illustrating they can control the violence. And overnight, the army cleared demonstrators from Parliament House after they used trucks to smash through its outer fence. But the army has made no arrests and a form of anarchy reigns on Islamabad's high-security Constitution Avenue, where crowds have been gathered for nearly three weeks.

A popularly elected government, which now also has the support of almost all opposition forces, is being cornered by a minority political group and the followers of a cleric who runs a charity network.

The government says its restrained response to rioters is meant to avoid bloodshed. But for many, Prime Minister Nawaz Sharif is on the back foot and short of options.

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.

Image copyright AFP Image caption Demonstrators have been taking part in a sit-in in the capital for two weeks

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. Three people died and hundreds were injured.

Last year's elections marked Pakistan's first civilian transfer of power. Mr Sharif won by a landslide and BBC correspondents say the vote was deemed to have been generally fair.