MOSCOW -- A subway train derailed Tuesday deep below Moscow streets, twisting and mangling crowded rail cars at the height of morning rush hour. At least 20 people were killed, Russian officials said, and 150 more were hospitalized, many with serious injuries.

The Russian capital's airports and transit systems have been a prime target for terrorists over the past two decades, but multiple officials vigorously dismissed terrorism as a possible cause.

The Moscow Metro is world-famous for its palatial interiors with mosaics, chandeliers and marble benches. Park Pobedy, where the derailment occurred, is Moscow's deepest metro station -- 84 metres below the surface -- which made the rescue particularly difficult. The station serves the vast park where Russia's World War II museum is located.

It was unclear what caused the train to derail. Lines of inquiry included a fault in one of the cars or the sinking of the roadbed, according to Vladimir Markin, spokesman for Russia's top investigative body. He said other officials who said earlier that a power surge triggered an alarm, causing the train to stop abruptly, were incorrect.

Of the 150 people reported injured, at least 50 were in grave condition, health officials said. One citizen of China and one citizen of Tajikistan were among those killed, Russian news agencies quoted city officials as saying.

Over 1,100 people were evacuated from the train, which was stuck between two stations, in a rescue operation that lasted at least seven hours.

By late afternoon, rescuers had recovered seven bodies and were working to extract 12 more trapped in two wrecked train cars, said Alexander Gavrilov, deputy chief of Moscow's emergency services. One woman taken from the scene died at a Moscow hospital.

In video released by the Emergency Situations Ministry, several wrecked train cars looked almost coiled, occupying the entire width of the tunnel. Workers were trying to force open the mangled doors of one car to retrieve bodies. Photos posted on social media sites showed passengers walking along the tracks in the dimly lit tunnel.

Mayor Sergei Sobyanin told reporters that unnamed officials will not only be fired but also charged with crimes, though he would not say what charges they might face.

Dozens of injured people were carried out of the station on stretchers. Paramedics carried one woman covered with a blanket to the lawn by the Triumphal Arch, which commemorates Russia's victory over Napoleon, and put her on a helicopter ambulance.

Several survivors sat on the sidewalk near the station's entrance in an apparent state of shock, drinking water supplied by authorities on a hot summer day.

One man with a bloody cut on his brow told Rossiya 24 television outside the station that he felt a jolt before the train stopped abruptly.

"There was smoke and we were trapped inside," he said. "It's a miracle we got out. I thought it was the end."

While technical glitches are regular occurrences in the Moscow Metro, the subway hasn't seen deadly accidents in decades.

Terrorism is another matter. More than 100 people have been killed in bombings on Moscow's subway trains or near stations since 2000, including two bomb blasts on the same day in 2010 that killed a total of 40 people.