Protesters demanding the fall of the Pakistani government blockaded parliament and key ministries Wednesday in the latest round of a week-long political drama that has shaken the restive nuclear-armed nation.

Cricketer-turned-politician Imran Khan and populist cleric Tahir-ul-Qadri had Tuesday led followers in a late-night march on the parliament building, the culmination of a week-long standoff with Prime Minister Nawaz Sharif.

Khan and Qadri say last year's general election that swept Sharif to power by a landslide was rigged, and they are demanding his resignation.

Qadri repeated his demand for Sharif to quit and install a "national government", and ordered his followers to stop lawmakers leaving a national assembly sitting called to debate the crisis.

His activists occupied the main entrances to parliament but MPs left the building by a back entrance without incident.

Elsewhere in Islamabad's high-security government "red zone", followers of Qadri's Pakistan Awami Tehreek movement blocked the entrance to an office complex housing numerous ministries.

Under the gaze of riot police, they said they would not allow any ministers or MPs to leave until Sharif quit.

The showdown has added to the sense of instability in a country struggling with a homegrown Taliban insurgency, a crippling power crisis and a sluggish economy.

Neither protest leader has shown any signs of backing down, despite repeated government offers of talks.

But on Wednesday the Supreme Court, which has played an influential role in Pakistani politics in recent years, ordered Khan and Qadri to appear on Thursday to explain themselves, a court official said.

The ruling came after petitions urging the court to restrain Khan and Qadri from "making illegal and unconstitutional demands", Kamran Murtaza, a senior lawyer, told AFP.

There had been fears the protesters' advance on parliament could trigger clashes, but riot police and other security forces looked on without intervening.

- 'Patience required' -

The crisis has raised fears that Pakistan's fragile democracy could be under threat of military intervention -- the country of 180 million people has seen three coups since its creation in 1947.

Rumours have abounded that elements within the influential military have been behind Khan and Qadri's moves, though the cleric and the interior minister have adamantly denied this.

On Tuesday Khan had threatened to break into the PM's official residence unless Sharif resigned, though Qadri distanced himself from the call, saying his supporters would maintain a peaceful sit-in until Sharif stepped down.

Early on Wednesday the army's chief spokesman called for dialogue.

"Situation requires patience, wisdom and sagacity from all stakeholders to resolve prevailing impasse through meaningful dialogue in larger national and public interest," General Asim Bajwa said through a recognised Twitter account.

- Testy relations -

Sharif has a history of testy relations with the military -- his second term as PM ended abruptly in 1999 when then-army chief Pervez Musharraf seized power in a coup.

His government is thought to have angered the military further by pursuing criminal cases against Musharraf dating back to his 1999-2008 rule, including treason charges.

Military analyst Ayesha Siddiqui warned that the situation was very precarious.

"From the military perspective, they have tried and tested Nawaz Sharif a third time and they feel disappointed. Why would they let him be?" she told AFP.

But Hamid Gul, the former head of the powerful Inter-Services Intelligence agency, said that despite the military's differences with Sharif, he thought they were unwilling to get involved.

"They (Khan and Qadri) are trying to drag the army into it, to pull the army, but the army is very reluctant," Gul told AFP, adding that the crisis would inevitably weaken Sharif.

"If Nawaz wants to stay in power he nas ho choice" but to listen to the army, Gul said.

The United States, Britain and the European Union have all voiced support for Pakistani democracy and urged the feuding sides to negotiate a way out of the impasse.

Last year's election, rated free and credible by international observers, was an important landmark for Pakistani democracy -- the first time one democratically elected government had completed its term and handed over power to another.

Sharif's Pakistan Muslim League-N party has accused Khan and Qadri of trying to derail democracy.