Jean-Luc Melenchon, a leftwing veteran who founded La France Insoumise, said the imprisonment of Ocalan “is unfair”, adding that he had been in jail almost as much as Nelson Mandela.

“For peace, it is necessary to release #Ocalan,” Melenchon said in a tweet on Tuesday.

France will vote over two rounds in April on whether to give President Emmanuel Macron a second five-year term or entrust a different candidate with the presidency.

The first round takes on April 10. Two candidates with the most votes will then face a showdown on April 24.

Separately, candidate Fabien Roussel, a journalist, said Ocalan had been imprisoned in defiance of international conventions.

“His release will open a path to peace in order to promote the progressive, feminist and ecological project of which the Kurds are carriers,” Roussel tweeted on Tuesday.

Ocalan is the founder and leader of the Kurdistan Workers Party (PKK) and has been imprisoned on an island near Istanbul since he was captured in 1999.

Ocalan, jailed on the island of Imrali, played a significant role in a peace process between Turkish authorities and the PKK. Those talks and a ceasefire broke down in 2015, unleashing some of the worst violence since the insurgency began.

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