Newspapers and business leaders have also criticised Khan's tactics, which on Sunday included a call for "civil disobedience".
With Khan looking isolated, on Monday PTI made a dramatic double roll of the dice to try to re-energise its campaign.

First the party announced it would resign all of its seats in parliament and three out of four provincial assemblies.

Then Khan pledged to lead the protesters in a march on Islamabad's high-security "red zone", which houses key buildings including parliament, the prime minister's house and numerous Western embassies.

The area has been cordoned off with shipping containers and heavily guarded by security personnel since the protests began, setting the stage for possible clashes if Khan goes through with his pledge.

At the protest site, where a few thousand demonstrators tried to find shelter from the hot sun, PTI activists struck a defiant tone.

"We will march and stage the next sit-in in front of the parliament house," said Bilal Arshad, 20.

"We are not ready to listen to anyone except our chairman. We just want the resignation of the Prime Minister Nawaz Sharif, who is a symbol of corruption and bad governance."

Fellow protester Naeem Kazmi said the crowd was ready to "reply in a fitting way" if violence erupted.

The government on Monday began efforts to find a negotiated resolution to the protests, and planning and development minister Ahsan Iqbal on Tuesday urged Khan to talk.

"The whole political leadership is united for this. All opposition parties are trying to engage them and find out a political solution," Iqbal told reporters.

"You have a few thousand crazy guys but 180 million people who have elected this government are guarantors of this government."

Last week Sharif tried to head off the protests by setting up a judicial commission to investigate rigging allegations, but Khan dismissed the proposal immediately.

The government has also set up a parliamentary committee to look at electoral reform.

At a joint press conference of all opposition parties except PTI, Khurshid Shah, a senior figure in the Pakistan People's Party (PPP) urged Khan to come to the negotiating table.

The general election of May 2013 which swept Sharif's Pakistan Muslim League-N (PML-N) party to power -- and brought PTI its best-ever result -- was rated as free and credible by international observers but both Khan and Qadri insist it was fixed.

The European Union issued a statement voicing its support for democracy in Pakistan -- and linking it to a highly-prized trade deal.

"The European Union Delegation underlines its conviction that the Constitution of Pakistan provides the legitimate framework for dialogue to resolve the current impasse," the statement said.

Enhanced trade privileges extended to Islamabad were "the ultimate reflection of the EU's strong commitment to the economic prosperity of a democratic Pakistan."

The PML-N has accused Khan of trying to derail Pakistan's perennially fragile democratic system.

The nuclear-armed country has experienced three military coups and the latest crisis has triggered more speculation about possible intervention by the powerful armed forces.