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The country's two main guerrilla forces, with a combined combat force of some 22,000 fighters, attacked police and army units, bombed banks and a major oil pipeline in at least nine of Colombia's 32 provinces, police and army sources said.

The Revolutionary Armed Forces of Colombia (FARC), the country's main guerrilla force, and the smaller National Liberation Army (ELN), launched their offensive early on Tuesday and clashes were reported throughout Wednesday.

The fighting, however, was well away from the colonial port city of Cartagena, where Clinton was meeting his Colombian counterpart President Andres Pastrana. The walled city, once Spain's biggest port in the New World, was heavily guarded by more than 5,000 soldiers, police and U.S. secret agents.

But Colombian police said they had discovered and deactivated a 4.4 pound (2 kg) bomb and arrested two alleged guerrilla fighters in a house in the northern port city, about 400 yards from a building Clinton had been due to visit. U.S. secret service agents said, however, there was no device but simply bomb-making equipment.

Rebels and grassroots organisations, including the main unions and student groups, condemned Clinton's visit saying a recently approved U.S. package of $1.3 billion in military aid would stoke Colombia's long-running war that has cost more than 35,000 lives in the last 10 years.

"The guerrillas are trying to paint a black picture of the situation here to coincide with Clinton's visit," a police spokesman said of the wave of rebel attacks.

In an effort to soothe fears of growing U.S. intervention in Colombia, Clinton said in Cartagena the U.S. aid would not create "another Vietnam" or give rise to "Yankee imperialism". The aid was designed to help Colombia fight the drug trade, which he said was in turn fuelling the civil conflict.

A police spokesman said of the 20 dead since early Tuesday, 11 were civilians, three were guerrillas and the rest police.

One of the worst single incidents took place in southwest Huila province when a FARC unit killed four policemen guarding a highway. FARC fighters also killed three workers on a banana plantation in the northwest Uraba region.

In Bogota, a riot policeman was killed when hooded students threw a home-made explosive during running battles outside the capital's National University.