Tens of thousands of anti-government protesters armed with wire cutters and backed by cranes have marched on Pakistan's parliament, removing barriers blocking them from soldiers guarding the seat of the country's government.

The protesters forced their way on Tuesday through barbed wire and road barriers blocking the way to Islamabad's "red zone", which also holds the president and prime minister's ceremonial homes and many diplomatic posts.

Police officers watched as an estimated 30,000 protesters moved into the government area, to back opposition politicians Imran Khan and Tahir ul-Qadri's call for the resignation of the prime minister, Nawaz Sharif.

Two Pakistani security officials told the AP news agency said that a total of 700 troops had also been deployed to guard the area, adding to an estimated 30,000 members of the country's security forces already there.

Authorities pleaded for calm ahead of the march, then later warned of possible bloodshed.

"They want blood. They want dead bodies. That's their politics,'' said Pervaiz Rashid, Pakistan's information minister. "If anything happens, the protesters will be held responsible."

The demonstrators have held two rallies in Islamabad in the last week. Khan and Qadri have vowed to keep up the sit-ins until Sharif resigns.

Khan, who heads parliament's third-largest party, announced on Monday that he and his supporters would march the government area, and said he expected a violent response from government forces.

"Let us promise that we will remain peaceful,'' Qadri asked his followers.

Khan's Tehrik-e-Insaf party has alleged that Sharif's Pakistan Muslim League-N rigged last year's elections. Sharif has agreed to set up a judicial commission to investigate the allegation but has refused to step down.

Khan has said he would not go home without Sharif's resignation.

The government has invoked a rarely used article in the constitution allowing the military to introduce martial law if needed.

Both Khan and Qadri have denied receiving support from sympathizers within the security forces. Relations between Sharif and the military frayed when the government decided late last year to prosecute Musharraf for high treason.

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Security forces had previously said they would not allow protesters to enter the "Red Zone" [AFP]

Aljazeera