MOSCOW - An explosion ripped through a subway car in the Moscow metro during morning rush hour Friday, killing at least 39 people, wounding dozens of others and sending clouds of smoke through the tunnel in what authorities said was a terrorist act.

Officials differed over whether the blast had been caused by a suicide bomber. Deputy Moscow Mayor Valery Shantsev said that investigators had not found metal shrapnel, which usually fills suicide bombers' explosives. He said that the bomb had likely been in a suitcase or rucksack on the floor of the subway car.

Passengers were evacuated through the dark tunnel to a station hundreds of yards from the damaged train, said Viktor Beltsov, spokesman for the Emergency Situations Ministry. He said there had been a fire, but other officials said there was no fire.

Deputy Interior Minister Alexander Chekalin said at least 39 people were killed, and 122 were hospitalized including one child.

More than 700 people were evacuated from two metro stations, ITAR-Tass reported. Most Muscovites are dependent on public transportation, and trains are usually packed during rush hour.

Viktor Habarov / Reuters Russian Emergencies Ministry specialists look into a damaged carriage on the Moscow subway.

Valery Draganov, a member of parliament representing the Avtozavodskaya district, told Echo of Moscow radio that body parts were scattered along the tracks. Inside the badly damaged wagon, bodies sat side-by-side still in their seats, and covered in soot.

Moscow police spokesman Kirill Mazurin said the bombing was being investigated as an act of terrorism -- the latest in a series of attacks that have plagued the Russian capital and southern cities.

The Interfax news agency, citing unnamed police sources, said the attack was carried out by a female suicide bomber. Police have a videotape of the suspected attacker and her alleged accomplice standing on the metro platform before boarding the train, Interfax reported.

The Federal Security Service was not ruling out that someone had been transporting explosives on the metro and they detonated prematurely, ITAR-Tass reported.

There were no claims of responsibility.

'Plague of the 21st century'
Russian President Vladimir Putin appealed to the international community to boost its efforts in the fight against terrorism, which he called "this plague of the 21st century."

Ambulances crowded outside the entrance to the Avtozavodskaya station, southeast of downtown Moscow. Rescue workers carrying empty stretchers or wearing equipment on their back rushed down the stairs into the station, and television networks showed footage of rescuers carrying a motionless body out on a stretcher. The injured were being sent to three Moscow hospitals.

The explosion occurred in the second car of a subway train after it had pulled about 1,640 feet from the Avtozavodskaya station and headed northwest to Paveletskaya station on the city's busy circle line, Mazurin told NTV television.

He told Rossiya television that after the explosion, the train traveled for about another 1,640 feet before coming to a stop. The line where the explosion occurred is one of Moscow's deepest.

Russia's Echo of Moscow radio, citing emergency workers, said that more than 150 people suffered from injuries including broken bones, smoke inhalation and burns.

"I heard a loud sound like a petard and smoke filled the car," said Ilya Blokhin, 31, a doctor who was on the train's next-to-last car -- several cars away from the blast. "Now that there are explosions on the metro, what are our country and government and police going to do when they blow up crowded subway cars?"

An unidentified woman, blood covering her face, told NTV television that for a long time after the explosion, passengers were unable to open the door of the subway car. After finally prying open the door, she said they walked a long distance out of the tunnel.

More than 700 evacuated
More than 700 people were evacuated, the ITAR-Tass news agency reported, citing metro staff. The majority of Russians are dependent on public transportation, and the spacious train wagons are usually packed tight during rush hour.

"It's scary to live here," said Galina Abramova, a passenger on a train that was coming in the opposite direction when the explosion occurred. "I wasn't that close to the train but I feel scared anyway ... now at noon, my feet are still weak."

Alexander Maximov, 18, said he heard a bang as he was descending into the Avtozavodskaya station.

"I feel scared, very scared," he said.

Police immediately barricaded the two metro stations closest to the train and stopped all traffic on the entire subway line. Dozens of buses were rerouted to deal with the passengers evacuated from the subway, clogging up the already jammed streets of the Russian capital.

Rossiya television said that Moscow Mayor Yuri Luzhkov was cutting a U.S. trip short to return to Moscow.

The U.S. Embassy said that it was in contact with Russian authorities to establish whether any Americans were among the victims.

Moscow on alert
The Russian capital has been on alert for terrorist attacks following a series of suicide bombings that officials have blamed on Chechen rebels.

In December, a female suicide bomber blew herself up outside the National Hotel across from Moscow's Red Square, killing at least five bystanders. Two suicide bombers blew themselves up at a Moscow rock concert last July, killing themselves and 14 other people.

Five days later, an aborted suicide bomb attack at a central Moscow restaurant killed a disposal expert who was trying to defuse the bomb. The woman who had carried the bomb was arrested and is currently awaiting trial.

A bombing in a Moscow subway car in June 1996 killed four people, and another injured at least nine in a busy metro station in February 2001. An abandoned handbag exploded on a subway platform on New Year's Day in 1998, wounding three metro employees.

In August 2000, a bomb exploded at a crowded pedestrian underpass filled with kiosks at Pushkin Square, a popular meeting place near several metro stations in the heart of Moscow. The attack was initially blamed on Chechen rebels, but some police later said that a turf battle between rival businessmen or criminal gangs could have been the motive.