A Turkish court on Monday, April 25 sentenced leading activist Osman Kavala to life in prison on controversial charges of trying to topple the government that had already seen him jailed without a conviction for more than four years, AFP reports.

The panel of three judges also jailed seven other defendants for 18 years each on the charge of aiding the attempt to overthrow then-prime minister Recep Tayyip Erdogan's government during large-scale protests in 2013.

The ruling drew swift condemnation from some of Turkey's main allies, as well rights campaigners -- some of whom emerged from the packed Istanbul courtroom in tears.

Washington said it was "deeply troubled" by the "unjust" conviction.

"The United States is deeply troubled and disappointed by the court's decision," US State Department spokesman Ned Price said in a statement.

Germany said the 64-year-old intellectual and campaigner must be "freed immediately", while two leading European parliamentarians who coordinate ties with Ankara said the "regrettable" ruling showed there was "little to no EU perspective for the current Turkey".

The bloc's chief diplomat, Josep Borrell, condemned the sentence for ignoring orders for Kavala's release from the European Court of Human Rights.

"Today, we have witnessed a travesty of justice of spectacular proportions," said Amnesty International's Europe director Nils Muiznieks.

The marathon hearing has been gnawing on Turkey's strategic but tempestuous ties with its main Western allies since Kavala's unexpected arrest in October 2017. Kavala was then best known as a soft-spoken businessman who was spending part of his wealth to promote culture and projects aimed at reconciling Turkey and its arch-nemesis Armenia.

But Erdogan portrayed him as a leftist agent of the Hungarian-born US billionaire George Soros and accused him of using foreign money to try and overthrow the state.