DUBAI, United Arab Emirates -- Bahrain's highest court on Monday upheld life sentences for a prominent Shiite cleric who led the country's now-outlawed opposition party and two of his colleagues after their earlier acquittal, continuing a yearslong crackdown on all dissent in the island nation.

Sheikh Ali Salman, a central figure in Bahrain's 2011 Arab Spring protests and the secretary-general of the barred Al-Wefaq political party, is already serving a nine-year prison sentence.

The Court of Cassation's ruling Monday ensures he won't be released and continues an imprisonment that United Nations experts have criticized as "a breach of his fundamental human rights."

The case against Salman and former Al-Wefaq lawmakers Sheikh Hassan Ali Juma Sultan and Ali Mahdi Ali al-Aswad, both of whom are abroad, involves allegations the men spied for Qatar amid the 2011 protests.

The charges came after Bahrain state television aired recorded telephone calls between Salman and Qatar's then-Prime Minister Sheikh Hamad bin Jassim Al Thani. It remains unclear who gave state television the recordings, though activists suspect the island's intelligence services leaked the call.

Bahrain, home to the U.S. Navy's 5th Fleet and a British naval base, is one of four Arab countries that have been boycotting Qatar since June 2017 as part of a wider diplomatic dispute.

The call between Salman and the Qatari official at the time was aimed at peacefully resolving the 2011 protests, which ended when Bahraini, Emirati and Saudi security forces violently put down the demonstrations. A government-sponsored report later noted Bahrain's opposition accepted Qatar's mediation while Bahrain's government rejected it.

A lower court initially acquitted the three men. Bahrain's Supreme Court of Appeals later overturned that verdict and found them guilty in November. The Court of Cassation's ruling Monday, reported by the state-run Bahrain News Agency, confirmed the verdict.

Activists immediately criticized the ruling.

"This is political revenge and an insult to justice," said Sayed Alwadaei, the director of advocacy at the Bahrain Institute of Rights and Democracy. "Punishing peaceful dissidents for leading protests against the corrupt ruling family has nothing to do with justice."

Amnesty International called for Salman's immediate release.

"Today's verdict is yet another nail in the coffin for the right to freedom of expression in Bahrain and exposes the country's justice system as a complete farce," said Samah Hadid, Amnesty's Middle East campaigns director.