The BBC's Paul Welsh: "Many see Nawaz Sharif as a political enemy"

A government spokesman said the bomb was set to explode when Mr Sharif - who uses the route at the same time every week - crossed en route to his nearby family estate.

Mushahid Hussain: "Reprehensible act of terrorism"

Information Minister Mushahid Hussain said the prime minister had a lucky escape, after work commitments delayed his departure from Lahore by 10 minutes.

The concrete bridge, less than two miles from Mr Sharif's private estate in Raiwind, was ripped apart by the blast. Two people died at the scene and a third died later.

Mushahid Hussain said: "The government has taken stringent measures to root out terrorism and this incident has further strengthened our determination to fight against this menace."

Speaking to the BBC World Service's Newshour programme he said: "It was a premeditated act directed at the prime minister. He was scheduled to pass [the bridge] on his usual route and usual time."

He said a review would be carried out of Mr Sharif's security arrangements.

Prime minister upbeat

He added that in a telephone call with Mr Sharif, the prime minister's morale was high.

Police inspect a tractor damaged by the bomb

Police were checking security along the road when the bomb exploded on Sunday.

Security across Lahore has been increased as investigators try to find the bombers.

Officials say it was a powerful timebomb which left a large section of the bridge badly damaged.

Correspondent Zaffar Abbas: "Sharif delayed by 10 minutes"

They have not indicated who may have been responsible for the attack and Pakistan has a history of bomb explosions without any group claiming responsibility.

Recently Mr Sharif hs cracked down on a number of armed religious and ethnic groups.