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

Government officials said earlier that the explosion was an attempt to assassinate the prime minister.

Three people were killed when the bomb exploded under a bridge on the outskirts of the city of Lahore shortly before Mr Sharif was due to pass by.

Mushahid Hussain: "Reprehensible act of terrorism"

Radio Pakistan reported that Mr Sharif and the Pakistani President, Mohammad Rafiq Tarar, condemned the bombing as a "heinous act of terrorism."

Mr Sharif later visited the site of the blast and ordered all necessary medical help for the injured.

Timebomb

Police inspect a tractor damaged by the bomb

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.

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.

Explosive history

Correspondent Zaffar Abbas: "Sharif delayed by 10 minutes"

Officials 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.