At least 21 people have been killed and more than 100 people injured, many of them seriously, after a Moscow subway train derailed during the busy morning rush hour.
A spokesman for the Emergency Situations Ministry initially said a power surge triggered an alarm, causing the train to stop abruptly and several cars to derail between the Slaviansky Boulevard and Park Pobedy stations.
A spokesman for Russia's Investigative Committee, Sergei Markin, later said an investigation has been launched into possible violations of transportation regulations.
Officials say they have ruled out terrorism as the cause of the accident, saying there is no evidence of an explosion and that all indications point to technical problems as the cause.
The driver of the train was among those killed.
Many of those injured are said to be in serious condition. Helicopters were used to evacuate the injured from the scene.
Yury Akimov, a Moscow representative of the emergency services, said in a televised briefing outside the Park Pobedy station that about 200 people were evacuated from the train.
Rescue workers said more than 1,000 people were evacuated from the scene of the accident.
WATCH: Emergency workers rush to help victims of a subway train derailment that killed at least 20 people and injured more than 100 in Moscow.
Park Pobedy is the deepest subway station in Moscow at 84 meters, complicating rescue efforts.
Moscow Mayor Sergei Sobyanin vowed to take the "toughest measures" against anyone who might have been in some way responsible for the accident.
Sobyanin said Moscow will observe a day of mourning on July 16.
Spokesman Dmitry Peskov said Russian President Vladimir Putin, who is currently in Brazil, had been informed of the situation.
The accident is one of the most serious to hit the Moscow subway in recent years.
The Moscow subway carries about 9 million passengers per day through the Russian capital.