At least seven were killed and many more injured in the blast

Police are blaming left-wing rebels for the blast, which happened at around 0940 local time (1440 GMT) in the city of Cucuta.

The bomb went off in the underground car park of the Alejandria Shopping Centre, a favourite with the city's poorer population, and ignited a huge fire that forced shoppers to flee in panic.

We heard a very strong explosion and then flames burst out. Then there was a fire and some parts of the building caved in Eyewitness

The blast brought terror to a city that has one of the highest murder rates in Colombia, itself one of the most dangerous countries in the world.

Rescue services, including some from just over the border in Venezuela, rushed to the scene as frightened passers-by looked on.

Firefighters fought the blaze, with many of the injuries coming from smoke inhalation as people struggled to get out.

The death toll rose after one of the floors of the shopping centre collapsed on rescue workers and civilians trying to help evacuate the building.

Wave of attacks

Initially, police believed they were dealing with a car bomb, but they later said it was an explosive device placed close to a parked car.

The governor of Norte de Santander province, Juan Santaella, said the blast was "a real tragedy" for the city, the French news agency AFP reported.

During the last few months, there has been a wave of bomb attacks in Cucuta.

The city lies some 450 kilometres (280 miles) north of the capital, Bogota, close to the border with Venezuela.

The area has become a frontline in the 39-year civil conflict fought by the state, Marxist guerrillas and right-wing paramilitaries and drug barons.

Local police believe the blast is the work of guerrillas from the National Liberation Army (ELN), although the larger Revolutionary Armed Forces of Colombia (FARC) also operate in the region.

Both groups are fighting right-wing paramilitaries for control of the drug crops which proliferate in the area.

Security

Whoever was behind the bombing, the BBC's Jeremy McDermott in Bogota says, it further undermines the security strategy of President Alvaro Uribe, who has pledged to crush the rebel insurgency.

He has relieved the Cucuta police chief of his command, saying there was evidence that elements within the police and the attorney general's office are infiltrated by rebels.

He sent the police general from Bogota to take charge of the situation and is demanding results.

President Uribe said it was imperative that Colombians and the world saw the rebels for what they were, not heroes or Robin Hood figures, but simply terrorists, our correspondent says.