CARACAS—Armed Venezuelan federal intelligence agents on Thursday detained Caracas Mayor Antonio Ledezma, a vocal critic of President Nicolás Maduro, who accused his opponent of unspecified “crimes against the country.”

In a televised address to supporters, Mr. Maduro didn’t offer evidence of the charges and Mr. Ledezma’s aides told reporters no arrest warrant was presented. The detention drew swift condemnation from the U.S. State Department and human rights groups.

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