Officials in Argentina say they have found explosives at a theatre in the capital, Buenos Aires, where former Colombian President Alvaro Uribe was due to make a speech on Wednesday.
Security personnel found the explosive device hidden in a lamp.
Officials said the bomb would have been triggered by a mobile phone.
Mr Uribe governed from 2002 to 2010, during which time he made the defeat of Colombia's largest rebel group, the Farc, his main priority.
The device was found in the Gran Rex Theatre during a routine inspection.
Judicial official Carlos Leiva told the Associated Press news agency that the bomb was hidden in the power supply for a ceiling light in the second floor, just where Mr Uribe was expected to meet business executives and other guests after his talk.
Federal Judge Norberto Oyarbide, who has been put in charge of the investigation, said the device was "simple, but sufficient to kill those nearest to the blast".
Judge Oyarbide said it had been set to explode on Wednesday at 16:30 (19:30 GMT) when a large number of people, including Mr Uribe, were due to attend a drinks reception on the second floor of the theatre.
Argentine police explosive experts are examining the building, and the street outside has been cordoned off.
The theatre personnel have been evacuated.
The device was found a week after another Colombian politician, Fernando Londono, was targeted in a bomb attack in the Colombian capital, Bogota.
Mr Londono, who served as justice and interior minister under Mr Uribe, was injured in the attack, which killed his security guard and his driver.
An unidentified man had attached a limpet mine to the door of Mr Londono's car, as it stopped at the lights in a central Bogota street.
Mr Londono has blamed left-wing Farc rebels, but Colombian President Juan Manuel Santos said he would await the outcome of the police investigation before attributing blame for that attack.
Mr Uribe has been praised for a series of successes against the Farc, the most high-profile of which was the 2008 military operation which resulted in the freeing of 15 hostages held by the Farc, including the French-Colombian politician, Ingrid Betancourt.
But Mr Uribe's hardline policy against the rebels also came under criticism, and a number of high-ranking members of the military have been jailed for human rights abuses.
Mr Uribe has been the target of several assassination attempts.
In April 2002, Farc rebels placed a bomb in a bus along the route Mr Uribe's campaign convoy was using in the Colombian city of Barranquilla.
The bomb went off, but the armour of Mr Uribe's vehicle saved him from harm.
Mr Uribe's father was killed by guerrillas in 1983 at the family ranch in Antioquia.