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By Patrick Markey

BOGOTA, Aug 15 (Reuters) - Seven people were killed and nearly 50 wounded in the most serious Colombian guerrilla attack this year when a bomb exploded in a small town as residents celebrated a festival, officials said on Friday.

Officials blamed the Thursday night explosion in Antioquia province on the Revolutionary Armed Forces of Colombia, or FARC, rebels who they said were retaliating for efforts to eradicate coca leaf plantations used to make cocaine.

The attack came at a time when guerrillas have been battered to their weakest in years by President Alvaro Uribe's U.S.-backed security campaign. But rebels remain a force in remoter areas, aided by profits from drug trafficking.

Television showed army helicopters ferrying the most seriously wounded out of Ituango, where families were celebrating a local festival and shopping at street stalls when the bomb exploded at around 10 p.m., authorities said.

"I was having a good time with friends. We were standing waiting for others to go to the store when we felt a huge explosion," victim Gilbert Alexis told local radio. "When I got up my legs were hurt. There were wounded people all around."

The attack came more than month after the Colombian army tricked rebels into handing over high-profile hostages, Ingrid Betancourt, and three captive U.S. contract workers, in one of the most serious blows to Latin America's oldest insurgency.

In Ituango, green and white plastic chairs lay scattered among rubble on the closed street, where the bomber left his explosive in a litter bin among the party-goers.

Uribe planned to fly to the site of the attack on Friday. Antioquia provincial Gov. Luis Alfredo Ramos said one suspected member of a local FARC group was captured after the bombing.

Ituango is in the remote northern part of Antioquia, where coca leaf is cultivated for drug traffickers. The region has seen recent protests against eradication efforts, which the government said were organized by the guerrillas.

"This can be attributed to the FARC's 18th front ... in retaliation against the campaign to eradicate illicit crops," said Antioquia police commander Col. Luis Eduardo Martinez.

Massacres, kidnappings and bombings from Colombia's four-decade-old conflict have waned as Uribe has sent troops to drive the FARC back into remote mountains and jungles and negotiated the surrender of right-wing death squads.

Three top FARC commanders have died this year and the guerrillas have been hit by desertions as the government offers rewards for fighters to surrender and the military's improved mobility and intelligence keeps them on the run.

The U.S. government has supplied Colombia with more than $5 billion in mostly military aid since 2000 to battle rebels and drug traffickers. But Colombia remains the world's No. 1 cocaine producer exporting at least 600 tonnes a year. (Reporting by Patrick Markey in Bogota)

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