Pakistan has deployed the army to guard the country's parliament in Islamabad and the government "red zone" area in the face of a planned mass anti-government protest, officials have said.

The interior minister, Chaudhury Nisar Ali Khan, made the announcement on Tuesday, as opposition politician Imran Khan vowed he and thousands of his supporters will march toward the government quarter in the coming hours.

"There will be four layers of security in the red zone with police in the front, followed by frontier constabulary, rangers and the army. The army would ensure security of the sensitive buildings in the red zone," the state-run Radio Pakistan quoted the interior minister as saying.

The government has said Khan and Tahir ul-Qadri - a Muslim leader who is leading a separate anti-government rally - are free to protest peacefully but will not be allowed to enter the red zone - home to many Western embassies, the supreme court and government ministries.

Qadri and Khan are demanding Nawaz Sharif, the prime minister, step down over alleged fraud in last year's election.

Pakistani television channels showed live footage of the troops taking up positions - a first time for a civilian government in Pakistan's history.

Qadri has said that his supporters have decided to march peacefully on parliament to try to force the prime minister to resign, the Reuters news agency reported.

Tahir ul-Qadri said a "people's parliament" has decided to march after he asked the crowd several times if they wanted "a true democracy" and "no terrorism" and they shouted "yes".

Khan has also said his supporters would march on parliament at any cost - even if it cost lives.

Qadri has tens of thousands of supporters in the capital. Most of the men carry stout sticks and some have masks to protect them from tear gas. Many women are also attending his rally.