The bodies of five people have been found in Moscow's sewer system and several others are missing after becoming trapped by heavy rain during a guided tour underground, the TASS state news agency reported.
Moscow Mayor Sergei Sobyanin called it a "terrible tragedy".
He said the search for missing people was continuing, and investigators were establishing details of what happened.
TASS said authorities had opened a criminal case and the organisers of the tour were being questioned.
It described the excursion as illegal.
Another agency, RIA, said eight people in total were believed to have taken part.
Video shared on social media showed divers searched the Moskva river.
Heavy rain in Moscow yesterday caused water levels in the sewage system to rise sharply, preventing the people from escaping to the surface, TASS quoted rescue workers as saying.
The dead included a company employee, his 15-year-old daughter and 17-year-old nephew, social media sites reported.
The reports have not been verified.
A number of guides offer tours of Moscow's sprawling sewer system, parts of which date back to the 19th century.
It is not clear if all of the visits are permitted by authorities.
Daniil Davydov, described as an "urban explorer", told RIA that there were shelters in the tunnel where people could escape, but that nobody could be found.
"I hoped that maybe I would still be able to find some survivors there. There are two shelter points, but there was no one there," he said.
