The bridge, two miles from Mr Sharif's private residence in Raiwind, was destroyed in the explosion, which was heard for miles around. The bomb went off around the time he and his family were expected to cross on their way from Lahore, the capital of Punjab, 13 miles away.
The Prime Minister's life was saved when he delayed his departure from his house in Lahore. He and his family were still there at the time of the blast and they later flew to Raiwind by helicopter.
The government called the explosion an "act of terrorism" while police said they believed that a political party formerly allied with Mr Sharif was behind the act. Three civilians and a police officer were killed, and three other policemen were wounded.
Police said they suspected the Muttahida Qami Movement, an Urdu-speaking party. There were reports that three MQM workers had been arrested in Karachi, more than 600 miles to the south, in Sindh province. Mr Sharif's Pakistan Muslim League has been at odds with the MQM for some time over the worsening chaos in Sindh province.
The government has blamed the MQM for inciting violence in Karachi, a city of 14 million, which has been controlled by the MQM since the mid- 1980s. More than 1,000 people died in political and religious violence there last year.
The MQM, which represents Urdu-speakers who fled the communal violence in India at the time of independence in 1947, denies the accusations and has accused the government of waging "terrorism" against its supporters.
In November last year, the government set up military courts to try people held responsible for violence in Karachi. They also suspended the provincial government of Sindh and the region is now ruled by the army and the federal government.
Officials said the Prime Minister was unruffled by the attack. "Our Prime Minister is a brave man," the Information minister, Mushahid Hussain, said. "I have just talked to him, he is unshaken. In fact he is more concerned about those who died and were wounded in the explosion."
Mr Sharif, 49, has emerged as the country's most powerful leader in decades. He became the first industrialist to head a Pakistani government in November 1990 after Benazir Bhutto was sacked by the former president Ghulam Ishaq Khan. In 1993, Sharif was also sacked by Ishaq Khan.
He was reinstated by the Supreme Court but later resigned in a deal that led to new elections, won by Bhutto. He then led his party to a landslide victory in 1997.Reuse content