Colombia's President Alvaro Uribe abruptly dropped a scheduled public appearance in the city of Medellin on Wednesday after reports that rebels were planning to assassinate him.

Mr Uribe - who has already survived 11 attempts on his life - was whisked away to an army base by the presidential protection team.

At the same time, the authorities announced that they had thwarted a devastating rebel attack on the capital, Bogota, after defusing a wave of powerful car bombs.

We have averted an unprecedented terror attack against Bogota Defence Minister Martha Lucia Ramirez

Police said they had arrested six suspected members of the Revolutionary Armed Forces of Colombia (FARC) in connection with the bombing campaign.

Mr Uribe was elected to office earlier this year promising to take a hard line against left-wing rebels and right-wing paramilitary groups.

Beforehand, there were several attempts on his life - the most serious when FARC guerrillas placed a bomb in a bus on the route Mr Uribe's convoy was taking.

In August, rebels attacked the presidential palace and congress with mortars as Mr Uribe was being sworn in.

Remote control

Officials said the rebels had apparently planned to attack Mr Uribe during his scheduled appearance at Medellin's Intercontinental Hotel.

The president, who was described as appearing unruffled by the turn of events, was flown to the base of the army's Fourth Brigade.

A bomb on Monday exploded near a police post

"You must never lose your tranquillity", he told reporters.

"But you must be prudent, since no-one is exempt from this nonsense."

The authorities announced on Wednesday that they had averted "a major terror attack" against Bogota.

"These five bombs could have done unimaginable damage," said Defence Minister Martha Lucia Ramirez.

In the last two days, five car bombs said to have been packed with 250 kilograms of explosive have been defused.

Police said the vehicles were equipped with remote control systems allowing them to be guided to targets believed to include supermarkets, bus stops and police barracks.

Civilian victims

Alvaro Uribe's father was killed by FARC rebels

On Monday, a car bomb went off in a car park at a grocery store, injuring more than 60 people.

The BBC's Jeremy McDermott says the FARC have long held a grudge against Mr Uribe since he was governor of the troubled province of Antioquia in the mid-1990s, accusing him of supporting right-wing death squads.

Thousands of people - mainly civilians - are killed each year in Colombia's long-running civil war which pits the government against the rebels and right-wing paramilitaries.