Pakistan crisis: Islamabad sees more violent protests
Clashes between security forces and protesters have continued for a third day in the Pakistani capital Islamabad.
The army has denied suggestions it is backing anti-government groups, insisting it is "apolitical".
Earlier Pakistan's national television channel was briefly taken off air after anti-government protesters stormed its headquarters in Islamabad.
Troops were sent in to regain control from demonstrators who had forced their way into the PTV offices.
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.
On Monday the army's public relations wing issued a statement in which it "categorically rejected" that it was supporting either Mr Khan or Mr Qadri.
In a statement, it said the army was "an apolitical institution and has expressed its unequivocal support for democracy at numerous occasions".
The statement comes after a senior figure in Mr Khan's PTI party, Javed Hashmi, claimed Mr Khan had told senior party members that the army and intelligence services were ready to help him and Mr Qadri topple the government.
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.
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.
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 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.
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.