A train derailment deep underground in the Moscow metro caused at least 20 deaths and 150 injuries during Tuesday morning's rush hour.

The accident occurred just after 9am in the west of the city between Park Pobedy and Slavyansky Bulvar stations, one of the deepest points in the Moscow metro system. Initial reports suggest a fire alarm system was automatically triggered in error, leading to the driver braking suddenly and one of the carriages coming off the tracks.

Later, however, a spokesman for Russian investigators said they were focusing on possible faults in the train cars or subsidence under the rails as possible causes. Terrorism was ruled out.

Witnesses reported that panic took hold as people were thrown around inside the carriages. The driver of the train was among the dead.

The Russian website LifeNews posted a video interview with a man who had been on the derailed train: "I got into the train, 20 seconds went by, then the lights went out and everything shook. I was thrown into the middle of the carriage. Panic broke out. Some men took the emergency hammers and smashed their way out. We were accompanied through the tunnel by construction workers. Some of the train was really smashed up, you could see a lot of people were crushed in."

Andrei Petrov, a passenger in the metro train behind the derailed train, said even in his carriage there were some injuries as the train braked suddenly.

"There was no panic, everyone waited in the carriage and waited for what the driver would say. But there was some injuries and blood, and one person broke a rib, I think," he told TV Rain.

Four hours after the incident deputy mayor Petr Biryukov said there were still seven passengers trapped inside one of the carriages underground, and rescuers were attempting to cut through metal in order to reach them. Rescuers worked throughout the day to free bodies from the wreckage. Carriages appeared to have come off the rails completely and turned on their side.

Health officials said at least 106 people were hospitalised, with around half of them in serious condition. Fire engines, ambulances and helicopters could be seen around the exits to the two stations, and several major roads were closed for the day to allow emergency vehicles to move unhindered.

The Moscow mayor, Sergei Sobyanin, said there would be a full investigation to determine whether there was any criminal responsibility for the incident. "We will get to the bottom of this situation and the guilty will be punished very harshly," he told Kommersant FM radio station.

He added that families of those who died in the tragedy would be paid 1 million roubles (£17,000), while the injured would receive half that.

The Moscow metro is one of the most efficient in the world, with trains running frequently and rarely stopping for signals. A journey costs 40 roubles (75p) in peak times when the trains are always rammed with commuters.

It is usually the easiest and safest way to get around Moscow, although the system has been targeted by Chechen terrorists before, most recently in 2010 when twin female suicide bombers killed more than 40 people in two separate attacks.

Russian authorities said there was no sign that Tuesday's incident was anything other than an accident.

Park Pobedy is one of the deepest stations on the metro system, which has complicated the rescue efforts. The escalators are over 100m long and take several minutes to reach the surface. Trains between the two stations will be suspended for at least two days, officials said.