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On a heavily guarded nine-hour trip to this Colombian port city, Clinton responded to concern in both Colombia and the United States that the mostly military aid package would result in a shooting war between Americans and drug traffickers or the Marxist guerrillas who protect them.

"We are not going to get into a shooting war. This is not Vietnam. Neither is it Yankee imperialism," Clinton said during a joint news conference with Colombian President Andres Pastrana.

In Bogota, as many as 5,000 protesters wearing Uncle Sam hats and skeleton masks shouted "Yankee go home!" and "Imperialism out of Colombia!" as they thronged outside the heavily fortified U.S. Embassy building.

At Bogota's National University, hooded students who also opposed Clinton's visit clashed with baton-wielding security forces in violence that killed an 18-year-old policeman and injured three people.

Elsewhere in the country, sporadic clashes were reported throughout the day. Since a wave of rebel attacks began on Tuesday, 11 civilians, three guerrillas and six police officers have been reported killed.

In Cartagena, Colombian police, acting on a tip, swooped on a house six blocks from a building Clinton visited later and found what they described as bomb-making materials, ammonium nitrate and black powder, U.S. Secret Service official Terry Samway said.

"There was no bomb, although there were materials that could conceivably be put together into one," Samway said.

TWO ARRESTED

A father and two sons at the home, believed to be sympathisers of Colombia's largest guerrilla force, the Revolutionary Armed Forces of Colombia, or the FARC, were arrested, Samway said.

American involvement in Vietnam began with a dispatch of military advisers.

But Clinton stressed that conditions attached to the aid package made it illegal for any U.S. troops to become involved in the anti-drug effort, apart from several hundred advisers who will train two Colombian army battalions. The plan also calls for the use of 60 U.S. attack helicopters.

The idea is for the battalions to protect police missions to destroy drug plantations and labs in guerrilla-controlled areas of southern Colombia.

The aid plan has been condemned by the country's powerful Marxist guerrillas and labour organisations, who fear it will stoke a three-decade-old war that has cost 35,000 lives.

Pastrana defended his $7.5 billion "Plan Colombia," which aims to destroy the Colombian drug market -- the world's leading supplier of cocaine -- while pouring money into social resources to help turn around the country's economy.

"Plan Colombia is not a plan for war. It is a plan for peace," he said.

Clinton made a "personal plea" to four nations bordering Colombia -- Ecuador, Peru, Brazil and Venezuela -- to support the anti-drug drive.