At least 11 children and an adult died when a storm caught a boating party on Lake Syamozero in northern Russia’s Karelia region.
Ten children on holiday from Moscow were killed, according to the mayor of the Russian capital, Sergei Sobyanin, quoting preliminary information.
According to an unconfirmed report, 36 members of the group of 49 survived and a search operation is continuing.
An instructor with the party was arrested as an inquiry began.
Russia’s Ria Novosti news agency quoted a source with rescue services as saying 25 survivors were on an island in the lake, and a further 11 were in a local village.
“A great tragedy has occurred in Karelia,” Mr Sobyanin tweeted.
“According to preliminary data, 10 children from Moscow have perished on Syamozero. My condolences to their families and friends.”