Pakistan police said they had arrested some 150 supporters of protest leaders overnight in Punjab province in a crackdown ahead of widening demonstrations in the capital.

As twin protests in Islamabad entered a fifth day on Tuesday, Pakistan police said they had arrested some 150 supporters of protest leaders overnight in Punjab province in a crackdown ahead of widening demonstrations in the capital.

Opposition leaders Imran Khan and Tahir ul-Qadri have led tens of thousands of supporters from Punjab to Islamabad in a bid to bring down Prime Minister Nawaz Sharif, whom they accuse of rigging elections and corruption.

Fearing any untoward incident taking place if the protesters enter the Red Zone, Corps Lieutenant General Qamar Bajwa and 10 other corps commanders have contacted top officials of the Islamabad Police for coordination to ensure security of the key government installations. The army is however not bound to act in aid of the Islamabad police in the enforcement of this section unless Chief Commissioner Islamabad orders them to do so.

In an earlier report, a military source put the number of troops stationed in the capital for security duties at about 350. But the city administration had told reporters that five army companies -over 500 soldiers- had been deployed.

The protests have piled extra pressure on the 15-month-old civilian government already struggling to overcome high unemployment, daily power cuts and a Taliban insurgency.

The protests have raised questions over the political stability of Pakistan, a nuclear-armed nation of 180 million people with a history of coups.

Police in Punjab, Pakistan's wealthiest and most populous province, arrested 147 supporters of Khan and Qadri overnight to prevent them joining the protests in Islamabad, said Inspector Rana Hussain in the central town of Sargodha.

Khan, a former international cricket star, said on Monday he would lead protesters into the capital's "Red Zone", an area home to Western embassies and key government ministries, on Tuesday.

The government has previously said protesters are not allowed to enter the area. It is flooded with riot police and paramilitary forces and cordoned off with shipping containers and barbed wire.

The government has not said whether Khan would be able to proceed to the Red Zone or not.

Qadri has said he will meet his supporters later on Tuesday to consider whether to march alongside Khan. Their protests have so far remained separate because the two have different supporters and plans for what should happen if Sharif steps down.

Sharif also has some support from the country's judiciary, which warned last week that the protesters should not consider any "unconstitutional" action.