Lawyers and human rights groups denounced the trial as a farcical attempt to prolong the detention of dissidents on trumped-up charges already found to be inadequate.

A well-known Turkish philanthropist went on trial again in Istanbul on Friday, his third prosecution in four years of detention, in a mass proceeding that has come to demonstrate the extreme lengths the Turkish government is willing to take to keep its opponents behind bars.

In a highly contested move, prosecutors merged the cases against three groups of defendants, most of whom have already been acquitted of any charges, to establish a new case against 52 people. The philanthropist, Osman Kavala, is the best known among the group, which includes football fans, environmentalists and artists who took part in the Taksim Square protests of 2013.

Mr. Kavala made a statement by video link from Silivri Prison, outside Istanbul, where he has been held mostly in solitary confinement for the past four years. A panel of judges ordered him to be further remanded in custody.

Charged with trying to overthrow the government and undermine the Constitution by violence, all of the defendants have long insisted they are innocent. The group also includes an American academic, Henri Barkey, who is accused of being in touch with Mr. Kavala at the time of an attempted coup in 2016. Mr. Barkey has denied any involvement in the event.

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