Guards foiled plans for a jailbreak similar to that shown in the film The Shawshank Redemption when they found a tunnel that inmates had dug, leading to a sewer.
Iris Varela, the prisons minister, said officials had been investigating a possible escape bid at La Planta prison in Caracas for three days before firefighters spotted the tunnel. She said the jail had installed bars in the sewers. The pipe emerged alongside a river.
Ms Varela said it was unlikely any prisoners had managed to escape before the tunnel was found.
Venezuela's jails were built to hold about 12,000 inmates but now hold about 47,000.