Pakistan's army chief General Raheel Sharif is mediating in a fortnight-long political crisis that has paralysed the government of Prime Minister Nawaz Sharif, opposition leaders said Thursday.

Populist cleric Tahir-ul-Qadri told thousands of followers camped outside the parliament building in Islamabad: "The Pakistan army chief has officially asked us if it will be acceptable to the Azadi (freedom) march if he becomes a mediator and guarantor."

"Do I have your consent?" he asked his audience, who loudly replied in affirmative.

He added the army had asked for "24 hours for mediation".

The protest's other leader, cricket hero turned politician Imran Khan, confirmed the move in a separate speech.

"General Raheel has asked us to resolve the issue through talks," he said.

"I want to tell you all that I will not disappoint you. The talks have already started," he added.

Government ministers and analysts have said that Pakistan's powerful army has been covertly aiding the two protest groups, who allege that last year's general election was massively rigged and want to topple the government.

The announcement that the military is acting as arbiter will further fuel speculation that the army could use the crisis to re-assert its dominance over civilian government, with which it has several disagreements.

Earlier in the day, Prime Minister Sharif had met the army chief to discuss the standoff, the second such meeting in three days.