Police said the bombers walked up to Pakistani army vehicles in the densely populated R A Bazaar area, blowing themselves up as people sat down to eat before the main Muslim weekly prayers were to begin.

The army cordoned off the tree-lined street where there were shops and a mosque, preventing access to journalists as ambulances raced through the city of 8 million to ferry the dead and wounded to hospitals.

The blasts underscored rising volatility in Lahore, where security forces have been a common target in attacks blamed on Taliban and Al-Qaeda-linked militants opposed to Pakistan’s alliance with the United States.

“The first blast was very small – followed by sounds of gunfire. Immediately after there was a big blast hitting an army vehicle,” said Mohammad Bilal, who had just sat down for lunch at a nearby restaurant.

“We have the heads of both the bombers,” police official Chaudhry Mohammad Shafiq told reporters.

“There was an interval of 15 seconds between the two attacks. They were on foot. Their target was army vehicles.

“At least 20 people were killed. Army personnel, were injured, some of them are in a serious condition.”

A wave of suicide and bomb attacks across Pakistan has killed more than 3,000 people since 2007. Blame has fallen on Taliban and Al-Qaeda-linked militants bitterly opposed to Pakistan’s alliance with the United States.

On Monday, a suicide car bomber destroyed security offices used to interrogate suspected militants in an upmarket Lahore neighbourhood, killing 15 people in an attack claimed by Pakistan’s mainstream Taliban faction.

Lahore, Pakistan’s historic cultural capital and home to many military and intelligence top brass, has been repeatedly in the militants’ sights, with eight attacks killing 155 people in the city over the past year.

Violence is usually concentrated largely in the lawless north-west border area with Afghanistan, but analysts have warned that extremism is taking a hold in Punjab, Pakistan’s most populous province which has Lahore as its capital.

Washington has put Pakistan on the front line of its war on Al-Qaeda, stepping up pressure on the military to act against Islamist militants and calling its border area with Afghanistan the most dangerous place on Earth.

Pakistan’s military claims to have made big gains against Taliban and Al-Qaeda strongholds over the past year, following major offensives in the north-western district of Swat and the tribal region of South Waziristan.

This year there had been a marked decline in violence by Islamist militants in Pakistan after a significant increase in bloodshed in late 2009.

Officials have linked the reduction to the suspected death – still not confirmed – of Pakistani Taliban leader Hakimullah Mehsud and military offensives that have disrupted militant networks.

Washington says militants in Pakistan’s semi-autonomous tribal belt are supporting the war in Afghanistan, where more than 120,000 NATO and US troops are battling a deadly Taliban insurgency.