Russian opposition leader Alexei Navalny attends a hearing after being detained at the protest against corruption and demanding the resignation of Prime Minister Dmitry Medvedev, at the Tverskoi court in Moscow, Russia in this file photo dated March 27, 2017. REUTERS/Tatyana Makeyeva

MOSCOW (Reuters) - Kremlin critic Alexei Navalny has undergone an eye operation in Spain after an attack in Moscow that left him almost blind in one eye, the opposition politician said late on Tuesday on social media.

Navalny hopes to run for president next year against Vladimir Putin, who is expected to run for what would be his fourth term, and organized the biggest anti-government protests in years in March.

The opposition politician, who opinion polls show currently trails Putin by a large margin, suffered a serious chemical burn to his eye after an assailant threw green liquid in his face on April 27.

Late on Tuesday, Navalny posted a photograph of himself on social media with his right eye looking bloodshot. He said he had undergone an operation in Barcelona and that a doctor there had told him his sight would only be restored “in several months.”

It had previously been unclear whether the Russian authorities would allow Navalny to travel abroad due to an embezzlement conviction against him in a case which he says was politically-motivated.

Navalny has called for large nationwide protests on June 12 to demonstrate against official corruption. He said on Wednesday that a presidential decree tightening the rules on holding such protests would not stop the planned events going ahead.