CARACAS — Clashes between armed inmates and guards in a crowded Venezuelan jail on Friday left at least 23 prisoners dead, a rights groups said.

The clashes started when inmates, many armed, refused to let the authorities enter, fearing they would be robbed, said Humberto Prado of the Venezuelan Prison Observatory.

At least 18 guards were injured, Mr. Prado said, adding that there were reports of at least one explosion.

The prison is in the central state of Portuguesa, 220 miles from Caracas. Designed to hold 250 people, it now bulges with 540 inmates, he said.