Thirteen children and one adult have died while rafting during a storm on Lake Syamozero in Russia's north-western region of Karelia near the Finnish border, emergency services told TASS news agency on Sunday.
Three boats full of 47 children from a summer camp and their four adult instructors had gone out Saturday on the lake, Investigations Committee official Vladimir Markin was quoted as saying.
The boats capsized in gale-force winds.
Local Culture Ministry official Yekaterina Biktimirova said the children were aged 12 to 15. "At night, they were caught in a storm. We already know about some victims," she added.
Fourteen bodies had been found, according to the emergency services, while the remaining members of the group had all been rescued.
Several helicopters had been sent to the site and more than 60 people had been involved in search operations, which had been called off Sunday evening.
The bodies and the survivors had all been taken to Karelia's capital Petrozavodsk.
Russia's Investigations Committee has detained one of the instructors and opened a criminal case, TASS reported.
Karelia has declared Monday a day of mourning.
Representatives of the Moscow authorities, psychologists and doctors were heading for Karelia, TASS reported.
"A big tragedy happened in Karelia," Moscow Mayor Sergei Sobyanin tweeted. "I offer condolences to relatives and friends."