Hundreds of protesters trying to topple Pakistan's government briefly seized the state broadcaster on Monday, intensifying the fortnight-long political crisis that has gripped the nuclear-armed nation.

Defence minister Khawaja Asif told AFP a cross-party negotiation team was set to approach opposition groups, in a bid to end a standoff that has seen three killed and hundreds injured in clashes between police and anti-government protesters.

Transmissions of the main Pakistani Television (PTV) news channel were cut after protesters armed with clubs stormed the building in Islamabad's high-security "red zone". They were removed by security forces after around half an hour.

The occupation came after fresh street clashes between riot police and followers of opposition politician Imran Khan and populist cleric Tahir-ul-Qadri, in which officers were pelted with rocks and responded with teargas.

Embattled Prime Minister Nawaz Sharif issued a brief statement dismissing reports of his imminent resignation, after holding talks with the powerful army chief and parliamentary leaders.

"The nation (can) be sure, I will neither resign nor go on leave," he said.

Khan and Qadri supporters have been protesting in the capital since August 15 to try to oust Sharif over alleged election fraud, triggering a crisis that has raised the spectre of military intervention in a country ruled for half its history by the army.

Late on Sunday the army called for a peaceful settlement, but warned it was "committed to playing its part in ensuring security of the state".

A senior figure in Khan's Pakistan Tehreek-e-Insaaf (PTI) party on Monday dramatically claimed the protests were being orchestrated by the military, echoing concerns voiced by analysts and government officials.

Asif, the defence minister, said a negotiation team drawn from various political parties was attempting to restart negotiations, though it was not immediately clear whether Khan and Qadri would accept further talks.

"The team consists of all parties. They will approach them tonight," Asif said.

Police in the eastern city of Lahore, meanwhile, added terrorism charges to a murder case registered against Sharif and 20 other senior government figures by the opposition -- ceding to a key demand by Qadri, according to the cleric's lawyer Mansoor Afridi.

- Military intervention? -

The crisis escalated on Saturday night when, after two weeks of charged but peaceful protests, violence erupted as followers of PTI and Qadri's Pakistan Awami Tehreek (PAT) tried to storm Sharif's official residence.

Protest leaders claim the 2013 general election which swept Sharif to power was rigged, even though local and foreign observers rated the polls as relatively fair and credible.

On Monday PTI president Javed Hashmi -- who publicly fell out with his party on Sunday -- controversially claimed the protests were being coordinated by the military.

He claimed Khan had said a military-backed plan was in place to bring an end to the crisis.

"Our leader told us that the matters have been settled and elections will take place in September," he told reporters.

"Imran Khan said we can't move ahead without (the) army."

PTI spokeswoman Shireen Mazari said Hashmi's allegations were "unfounded", while the army also issued a rebuttal.

"The army is an apolitical institution and has expressed its unequivocal support for democracy at numerous occasions. It is unfortunate that army is being dragged into such controversies," its publicity wing said in a statement.

- Rocks and tear gas -

Monday's clashes began as heavy rain fell on Islamabad, with more than 3,000 demonstrators trying to march once again on Sharif's residence.

Protesters pelted riot police with stones and some smashed up motorbikes with wooden clubs. Police tried to respond with tear gas, but the rain appeared to render it ineffective.

An AFP reporter saw more than 300 protesters, many armed with wooden clubs, enter the PTV building around 11:00 am (0600 GMT) shouting anti-government slogans.

Television footage showed some of them beating a photo of Sharif with sticks and spitting on it.

Army and paramilitary forces cleared the building without violence -- they were even cheered and applauded by the protesters as they left.

Several people at the scene were wearing PTI scarves and t-shirts, and Khan later apologised, saying his party workers had "become emotional".

The protest leaders have drawn thousands to the streets of Islamabad, but their call has not mobilised mass support in a country of 180 million people.

Asif, the defence minister, said: "We have a problem in that we have to get Constitution Avenue (the protest zone) vacated. The process may take time but time is not running out -- we're not on our way out.

"Except for these two guys and some thousands of people following them, every institution is with us."