Pakistani opposition politician Imran Khan Thursday called off talks with the government aimed at ending protests seeking the fall of the prime minister, which have unnerved the nuclear-armed nation.

Khan and populist cleric Tahir-ul-Qadri have led followers protesting outside parliament for the past two days demanding Prime Minister Nawaz Sharif quit.

Talks to end the standoff -- which began a week ago with "long marches" from the eastern city of Lahore -- started on Wednesday but made little headway.

Khan insists the May 2013 general election, won in a landslide by Sharif's Pakistan Muslim League-N (PML-N) party, was rigged, despite independent international observers judging it free and credible.

The former cricket star has demanded Sharif resign and call new elections and on Thursday repeated his insistence the PM must quit before talks.

"The talks with the government are over. How can these talks proceed when we first want resignation of Prime Minister Nawaz Sharif?" Khan said.

"I want to tell you Nawaz Sharif that I will not leave this place until you step down."

Between them, the protest rallies of Khan's Pakistan Tehreek-e-Insaf (PTI) and Qadri's Pakistan Awami Tehreek (PAT) have drawn tens of thousands of supporters this week.

But as Khan made his address from the top of a shipping container outside parliament on Thursday, there were barely a few thousand supporters scattered over the protest site.

PAT has said it wants "meaningful dialogue" and the powerful army has called for a negotiated end to the showdown.

The head of the Islamabad police was unexpectedly replaced on Thursday, officials said. Qadri said this was because he refused to open fire on the protesters.

Defence Minister Khawaja Muhammad Asif denied suggestions a crackdown on the protests was imminent.

"We have let them come for the past eight-nine days -- why would we crack down now?" he told reporters in parliament.

- Increased instability -

"We did not stop them in Lahore we did not stop them in Aapbara," he said. Aabpara is an area of Islamabad where the protesters first rallied.

Chief of religious Jamat-e-Islami (JI) party Siraj-ul-Haq called for an end to the crisis through talks.

"We appeal to all the parties to exercise restraint and avoid any deadlock in talks to resolve the crisis," he told reporters in Islamabad on Thursday.

Ahsan Iqbal, federal minister and a member of government team for talks with Imran Khan, told AFP "we are still hoping negotiations will resume."

A number of Pakistan's key trade partners -- and aid donors -- including the United States, Britain and the European Union have also urged for a political solution.

The showdown has added to instability in a country that has had three military coups since its creation in 1947 and which is struggling with a homegrown Taliban insurgency, a crippling power crisis and a sluggish economy.

The two protest movements are not formally allied and have different goals, beyond toppling the government. But their combined pressure -- and numbers -- have given extra heft to the rallies.

If PAT were to reach a settlement with the government and withdraw, PTI's position would be significantly weakened, despite Khan's tough stance.

Neither movement has mobilised mass support beyond their core followers and opposition parties have shunned Khan's call to unseat the government and begin a campaign of civil disobedience.

On Thursday the lower house of parliament passed a unanimous resolution rejecting calls for Sharif's resignation and vowing to uphold democracy.

Lawyers also staged a national strike in protest at the PAT and PTI sit-ins.

"This style of politics and call for disobedience is harming Pakistan's economy, a call for civil disobedience is equal to a constitutional coup," Yaseen Azad, senior leader, Supreme Court Bar Association, told Geo TV.