The explosion in Medellin also injured 38 people, destroyed the headquarters of the army's special anti-kidnapping unit and damaged nearby houses.

The BBC's Jeremy McDermott: "Medellin is no stranger to urban terrorism"

But as police continued to investigate the blast a second powerful explosion shook the centre of Manizales, a coffee-producing city in western Colombia.

Police were unable to confirm whether anyone had been hurt or killed by the blast, which occurred just before 1400 (1900 GMT) on Saturday.

Rebels blamed

Many of the wounded in Medellin - among them soldiers and passing civilians - are in critical condition.

The bomb - a truck packed with at least 200 pounds (90 kilograms) of explosives - went off at 1515 GMT in a residential neighbourhood.

The Colombian Army blamed the blast on the leftist Revolutionary Armed Forces of Colombia (FARC), one of the nation's most powerful rebel groups.

Passers-by were among those killed and wounded

It said the bomb was probably planted in retaliation for the arrest of seven rebels earlier on Friday.

"We have clear indications from our intelligence that show that it was the FARC's popular militias," said General Eduardo Herrera, commander of the army's Fourth Brigade.

Police spokesman Haten Dasuki said: "It was a powerful bomb, like the ones that went off during Pablo Escobar's time.''

During the 1980s, Medellin was constantly rocked by bomb explosions, as the now-defunct Medellin cocaine cartel led by Escobar waged a violent campaign against the government.

Rebel threats

The explosion destroyed the offices of the anti-kidnapping squad known by its Spanish initials, GAULA.

The director of Colombia's secret police (DAS), Guillermo Diaz, said the unit had been the target of several rebel threats in the past.

FARC rebels are in open control of some southern towns

The dead reportedly include a female passer-by, two officials from the federal prosecutor's office, and an agent from the state security police.

General Herrera said at least seven of the wounded were soldiers.

Television images showed the shattered facades of at least six houses, and vehicles flipped onto their sides.

The blast came as government peace envoy Victor Ricardo met FARC guerrillas in southern Colombia in an attempt to restart peace talks to end the long-running war.

Kidnappings

On 19 July, a car packed with explosives was set off by suspected leftist rebels in front of the offices of a large cement company in Medellin.

There were no deaths, but a night watchman was injured.

Colombia has the world's highest kidnapping rate, with 2,216 people abducted last year, according to the Pais Libre foundation, a private anti-kidnapping group.