The bomb was detonated by remote control
The blast is believed to be the third assassination attempt on the hardline right-winger who looks set to win Colombia's leadership elections next month.
At least 13 people were injured when the roadside bomb was detonated by remote control as Mr Uribe Velez's armoured car left a campaign stop at a public market in the Caribbean coast city.
The BBC correspondent in Colombia's capital, Bogota, said suspicion for the attack has fallen immediately on the Revolutionary Armed Forces of Colombia (FARC) which detests Mr Uribe Velez and has twice tried to kill him.
Initially police had said the bomb had been hidden on a bus.
But they later said the bus had been caught in the blast when a bomb under a bridge was detonated as Mr Uribe's convoy drove over it.
The candidate appeared on television shortly after the attack to say he was unhurt.
"It must have been a very powerful explosive because I was travelling in the most heavily armoured kind of vehicle, and the vehicle was destroyed," he said.
"But thank God, here we are, talking."
Politicians kidnapped
The attack came three days after suspected FARC guerrilla kidnapped 12 state legislators from an assembly meeting in the western city of Cali by posing as bomb squad officers helping to evacuate them.
Troops are still looking for the politicians, believed to be being held in mountains outside the city.
Violence has been rising in Colombia since February when the government ended a three-year peace process with FARC.
He said: "Colombia needs governments with authority to rein in violence.
"This is what happens to Colombians every day."
He has threatened an all-out war against the rebels should he become president.
Last January police arrested two snipers in the northern city of Medellin who said they were planning to assassinate Mr Uribe Velez.
In September, four alleged FARC members were arrested in a plot to put a suitcase containing Bibles stuffed with explosives at a favourite meeting place of Mr Uribe Velez in Bogota.
Running for president of Colombia can be deadly. During the 1990 campaign, three candidates were assassinated by drugs traffickers and right-wing paramilitaries.
Mr Uribe Velez, a 49-year-old Oxford and Harvard-educated lawyer, claims there have been 15 attempts on his life. He employs 120 bodyguards.
The latest opinion poll showed support for Mr Uribe Velez at 51% - a 22-point lead over his nearest rival.