BOGOTA, COLOMBIA, MAY 24 -- Terrorist attacks in Medellin three days before a presidential election left at least eight people, including three policemen, dead today and 21 injured, authorities said.

With three presidential candidates already assassinated, authorities have redoubled security measures across the country -- especially in Medellin, center of operations of cocaine trafficker Pablo Escobar, who is believed to be responsible for ordering the recent wave of violence that has wracked the country.

A car bomb explosion killed seven people, including the driver of the car and two policemen. The blast, near the Intercontinental Hotel on the southern edge of the city, injured 21, including 10 police officers.

Capt. Carlos Correa said in an interview with Caracol Radio that the car was stopped at a checkpoint manned by the elite corps of the National Police. The driver set off the bomb by remote control, he said.

Correa said, "The bodies were so badly destroyed and spread out over such a large area, we are not yet sure how many people we have."

In a separate incident, several minutes before the car bomb exploded, another policeman was killed in a shootout near the ruling Liberal Party's headquarters in downtown Medellin. Earlier in the day, a bomb virtually destroyed the studios of Colmundo Radio. There were no injuries.

On Monday, Federico Velez, a Liberal senator and supporter of presidential frontrunner Cesar Gaviria, was gunned down in Medellin.

In Bogota today, police, acting on intelligence tips that the Medellin drug cartel was moving a large number of hired killers into the capital, raided scores of low-class hotels where the gunmen were supposed to be staying.

An official report was not available, but police sources said dozens of young men had been arrested and an unspecified number of weapons seized. The government has warned that the cartel would seek to strike a spectacular blow before the election to destabilize the country and force the suspension of elections.