Update | More than 20,000 rally in Islamabad, calling for Sharif to resign as PM

More than 20,000 anti-government protesters flooded the centre of Pakistan's capital yesterday, vowing to stay in the streets until Prime Minister Nawaz Sharif resigns.

The numbers were far below what protest organisers expected, but the power of protesters to paralyse the central business district has presented the biggest challenge yet to the 15-month-old civilian government.

The unrest has raised questions about Pakistan's stability, at a time when the nation of 180 million is waging an offensive against Pakistani Taliban militants and when the influence of anti-Western and sectarian groups is growing.

Riot police cordoned off two streets in downtown Islamabad with shipping containers and barbed wire for the protests.

"We want Pakistan to be a peaceful state through our democratic revolution," populist cleric Tahir ul-Qadri told his followers in a short speech.

His supporters were adamant they would not leave until Qadri told them to. Most of the men carried stout sticks more than a metre long. Brigades of men and women in fluorescent jackets had gas masks, swimming goggles and bottles of water. "We are here to disarm the gas shells," explained one of the protesters.