ISTANBUL, Oct 8 (Reuters) - A Turkish court on Friday opened the re-trial of philanthropist Osman Kavala and 50 other defendants over nationwide protests in 2013, in a case which has been a source of growing concern among Turkey"s Western allies. Human rights groups have described the Gezi trial as symbolic of a crackdown on dissent under President Tayyip Erdogan over the last decade. The latest trial combines two cases related to the Gezi protests after their acquittals were overturned earlier this year. Hundreds of thousands marched in Istanbul and elsewhere in Turkey in 2013 as demonstrations against plans to build a replica Ottoman barracks in the city"s Gezi Park grew into nationwide protests against Erdogan"s government. The acquittals in the case targeting Kavala were overturned in January, and later combined with the trial against the Carsi football fan group over the same protests. read more The defendants in the Carsi case were acquitted in 2015 and that ruling was overturned this year. Some of the defendants in the Gezi trial are being tried over the same charges for a third time, having also been acquitted of them in 2015. Kavala has been jailed for four years without conviction despite the European Court of Human Rights (ECHR) calling for his release. The Council of Europe said last month it would begin infringement proceedings against Turkey if he is not released in line with the ECHR ruling. read more A separate case against Kavala and a second defendant over their alleged involvement in a 2016 coup attempt was also combined with the two other cases. "MISSED OPPORTUNITY" Around 35 defendants were present in the courtroom packed with Western diplomats, opposition parliamentarians and observers on Friday. The European Parliament"s Turkey rapporteur Nacho Sanchez Amor said he attended the session to show solidarity. "Sadly it was another missed opportunity for (Turkish) authorities to respect their intl" commitments," he said on Twitter. Speaking via videolink from prison, Kavala said the merging of the cases indicated a "politically tainted judiciary undertaking to prolong my imprisonment", adding that the allegations were based on conspiracy theories. The judge ruled that Kavala should remain in detention until the next session on Nov. 26. Critics say Turkey"s judiciary has been exploited to punish Erdogan"s perceived opponents. The president and his AK Party say the courts make independent decisions.