President Dmitry Medvedev has in a rare move vetoed a bill agreed by the Russian parliament last month which activists said would further restrict opposition protests, the Kremlin said on Saturday.
The new law was agreed by the State Duma, the lower house, on October 22 and then approved by the Federation Council upper house on October 27 and only needed to be signed by Medvedev to come into force.
The changes would notably have forbidden anyone convicted in the past of organising an illegal demonstration of putting in a request for permission to hold a protest.
"I reject the law," Medvedev said in a letter to the State Duma speaker Boris Gryzlov and the head of the Federation Council Sergei Mironov.
"The law... has aspects which would impede the realisation of the constitutional right of citizens to hold gatherings, meetings, demonstrations, marches and pickets," he said in the document released by the Kremlin.
The law had been passed overwhelmingly in the State Duma and was backed by the ruling United Russia party whose overall leader is Prime Minister Vladimir Putin.
Russian police have regularly cracked down on opposition demonstrations deemed to be unauthorised although some observers have detected a new climate of tolerance after the ousting of Moscow mayor Yuri Luzhkov in September.