Spanish Prime Minister Mariano Rajoy has added his voice to the international condemnation over Venezuela's arrest of Antonio Ledezma and urged authorities to free the Caracas mayor "immediately."

Ledezma was arrested last month on charges of plotting with the U.S. to overthrow Venezuelan President Nicolas Maduro.

Rajoy met with the Caracas mayor's wife, Mitzy Capriles, in Madrid on Thursday and called on Venezuela to release Ledezma and all other jailed opposition leaders.

Ledezma denies the accusations against him, as does Washington. Maduro's government has yet to present convincing evidence of the purported plot.

The mayor's arrest provoked spontaneous demonstrations in Venezuela, where the opposition has warned of an anti-democratic crackdown.