Four small bombs exploded in Colombia in what police said may have been a protest by leftist guerrillas against the presence of U.S. President Barack Obama in the country for the Summit of the Americas.

A senior police source in the capital Bogota told Reuters that two explosives were placed in a ditch in a residential area near the attorney general's office and the U.S. Embassy.

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"There are windows broken, but nobody hurt or killed," the source added.

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Obama landed in Cartagena, on Colombia's northern coast far from Bogota, earlier on Friday. "It might be a protest by an urban guerrilla cell against Obama," the source told Reuters.

Local authorities in Cartagena told NBC News that two small bombs also went off in Cartagena, about a 45-minute drive away from the summit site itself.

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