A suitcase bomb has exploded near a town square where 5,000 children were celebrating Halloween, killing two suspected bombers and injuring 37 people.
The wounded included two boys who were hospitalised in critical condition on Thursday, authorities said.
The two dead were suspected members of a drug-trafficking band allied with rebels from the leftist Revolutionary Armed Forces of Colombia, or FARC, said the regional police chief, Colonel Nelson Ramirez.
They were carrying the suitcase on a bicycle two blocks from the central square of Pradera, in Valle del Cauca state, where thousands of children had gathered, said town security chief Cesar Leyton.
The bomb apparently went off prematurely.
He said seven of the injured were hospitalised, including the nine and 11-year-old boys who suffered head injuries and were rushed in critical condition to the regional capital of Cali. Fourteen children were reported injured in all.
"It was chaos, horrible," said Alba Nelly, a 45-year-old housewife who was lightly injured in the blast, which struck as she sat by her door diagonal to the square with her 22-year-old paraplegic daughter.
"We were celebrating Halloween and my daughter was giving out candy to people who passed by," she told the Associated Press news agency by phone.
Ramirez speculated that the bomb's intended target was Pradera's police station, which is a block from where it exploded. He said such bombs are typically triggered by cellphones.
Pradera, a town of 60,000 people, is located in an area of traditionally strong FARC influence, where security forces are often attacked.
For years, the FARC sought a military withdrawal from the area in order to negotiate a prisoner swap. But successive governments refused.
The government of President Juan Manuel Santos formally inaugurated peace talks with the FARC on October 18 in Norway, seeking to end a stubborn nearly half-century conflict that yearly claims at least 3,000 lives.
The talks are to resume in mid-November in Cuba. The government has refused to agree to a cease-fire or a safe haven, which it granted during the last attempt to negotiate peace from 1999-2002.