Venezuelan President Nicolas Maduro has vetoed legislation that would give amnesty to activists the opposition considers political prisoners.

The opposition-controlled congress passed the measure last week calling for the release of dozens of people imprisoned in connection with opposing the socialist administration launched 17 years ago by the late President Hugo Chavez.

Maduro has called amnesty an attempt to free murderers, and on Thursday he officially rejected the bill. He also said he had asked Venezuela's Supreme Court to declare it unconstitutional.

The Supreme Court has blocked the National Assembly at every turn since the opposition took over the body in January and the justices are expected to kill the legislation as well.

The amnesty push has been one of the opposition's most important initiatives.