A Pakistani cricketer-turned-politician today called on thousands of anti-government protesters to stop paying taxes and practice civil disobedience until prime minister Nawaz Sharif steps down, raising fears of instability in the nuclear-armed US ally.

Imran Khan, who heads parliament's third largest bloc, made the announcement at a rally in the capital Islamabad calling for Mr Sharif to step down over alleged voting fraud in the May 2013 election, the first democratic transfer of power in a country with a long history of military dictatorships.

Mr Khan also warned that his supporters would take over parliament if Mr Sharif does not resign within two days.

"We decide today that we will not pay taxes to his illegitimate government, we will not pay electricity bills, gas bills," Mr Khan said to a charged crowd estimated at 10,000 to 15,000 people.

"I urge all the traders to stop paying taxes."

Mr Khan and cleric Tahir-ul-Qadri, a dual Canadian citizen with a wide following who spends most of his time abroad, have mounted twin protests that have brought thousands of people into the streets in Islamabad They accuse Mr Sharif of rigging the election that brought him to power.

Finance minister and Sharif ally Ishaq Dar called Mr Khan's move unconstitutional and information minister Pervaiz Rashid called the demand a "joke".

But both ministers told SAMAA TV that their government was ready to negotiate with Mr Khan over his demands for electoral reforms within the constitutional framework.

While the crowds have fallen well short of the million marchers that both men promised, their presence and the government's heightened security measures have virtually shut down business in the capital.

Police estimate that the crowds in both sit-ins have gradually dwindled since they arrived in the capital late on Friday.

Both rallies began as a caravan of vehicles setting out from the eastern city of Lahore.

Police official Nasir Shah estimated that there were currently about 25,000 to 30,000 people in both rallies.

Mr Sharif has given no indication he intends to step down.

Interior minister Nisar Ali Khan said the Pakistani government was setting up two separate committees of party leaders to open negotiations with Mr Khan and Mr Qadri.

"We are ready to accept all of your constitutional and legal demands," he said.