ISLAMABAD - Thousands of anti-government protesters armed with wire cutters and backed by cranes marched on Pakistan's parliament Thursday, planning to remove barriers blocking them from soldiers guarding the seat of the country's government.

Pakistani television channels showed live footage of the troops taking positions in the so-called "Red Zone," which also holds the president and prime minister's ceremonial homes and many diplomatic posts. That set up a possible showdown between hundreds of soldiers and some 30,000 protesters backing opposition candidate Imran Khan and anti-government cleric Tahir-ul-Qadri.

Both men have called on by Prime Minister Nawaz Sharif to step down over allegations of fraud in last year's election. Sharif has refused and ordered the soldiers out into the streets, the first military deployment in the capital since Pakistan has been under civilian leadership.

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Two Pakistani security officials said a total of 700 troops had been deployed to guard the "Red Zone." They spoke on condition of anonymity because they were not allowed to speak publicly about troop movements. Another 30,000 members of the country's security forces also were in the area, authorities said, as shipping containers and barbed wire block many roads.

A senior Pakistani foreign ministry official who spoke to CBS News' Farhan Bokhari on condition of anonymity separately said: "The army's deployment is largely meant to reassure members of the Western diplomatic community (which is based in Islamabad). Otherwise, many (western) governments may have immediately vacated their embassies without the army's protection."

Pakistan's Interior Minister Nisar Ali Khan pleaded for calm ahead of the march.

"Violence can't be allowed to happen," he said. "What is this all we are showing to the world?"

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

Khan, who heads parliament's third-largest political bloc, announced Monday that he and his supporters would march into Islamabad's "Red Zone." On Tuesday, Khan said they would make a "Tahrir Square" outside the parliament, referring to the famed square in Egypt that saw mass protests in 2011.

"Let us promise that we will remain peaceful," Qadri asked his followers. "No one will trespass into any building," Khan told his supporters.

Women and children also joined the protesters as the surged toward parliament.

Authorities have said they would not allow protesters to enter the "Red Zone" and asked the two opposition leaders to reconsider.

Amid the growing tension Tuesday, Pakistan's army chief Gen. Raheel Sharif also met with the prime minister in Islamabad, said Khan, the interior minister.

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

Khan has said he won't go home without Sharif's resignation, setting up a possible violent confrontation.

The standoff has raised fears of political instability in this nuclear-armed country of 180 million, which has largely been ruled by dictators since 1947.

Sharif, himself overthrown in the 1999 coup that brought former army chief Pervez Musharraf to power, has been meeting with top advisers ahead of the rally. The government also has invoked a rarely used article in the constitution allowing the military to introduce martial law if needed.

In recent days, analysts have warned that the use of force by Sharif's government and possible casualties among protestors could cause further instability across Pakistan, CBS News' Bokhari reports.

The Islamabad rallies come as Pakistan's military is waging a major operation against local and foreign militants in the North Waziristan tribal region bordering Afghanistan. The military said it carried our multiple airstrikes Tuesday in the Khyber and North Wazristan tribal region that killed 48 militants.

Pakistan's tribal regions are off-limits to journalists, making verifying claims over airstrikes difficult.

Also Tuesday, a roadside bomb struck a school van with children and teachers in Khar, the main town in the northwestern tribal region of Bajur, killing five people, a government administrator said.

No group immediately claimed responsibility for the attack. The Bajur tribal region has seen several military offensives against the Pakistani Taliban and foreign militants in recent years.