Russian police have detained more than 100 people at spontaneous memorials for deceased opposition leader Alexei Navalny, the OVD-Info rights group said on Saturday.

The 47-year-old Kremlin critic was serving a 19-year prison sentence in the Arctic when authorities announced his death, prompting grief among his supporters.

People were seen gathering to place flowers at makeshift monuments across Russia late Friday, and in some cases were detained by police, social media footage showed.

As of February 17, ‘more than 101 people’ had already been detained in 10 cities, including 64 in Russia’s second largest city of Saint Petersburg, OVD-Info said.

Eleven people were detained in the capital Moscow, and multiple others in the cities of Nizhny Novgorod, Krasnodar, Rostov-on-Don and Tver, it added.

Protests are illegal in Russia under strict anti-dissent laws, and authorities have clamped down particularly harshly on rallies in support of Navalny.

Authorities in the Russian capital said Friday they were aware of calls online ‘to take part in a mass rally in the centre of Moscow’ and warned people against attending.