ST. PETERSBURG, Russia, April 3 (Xinhua) -- Russian Health Minister Veronika Skvortsova said at least 10 people had been killed and 37 others were injured in an explosion of an unknown explosive device with destructive elements in St. Petersburg.

The explosion took place around 14:40 in a train carriage on the runway between the metro stations Technological Institute and Sennaya Ploshchad (Square) in Line 2 in St. Petersburg, the second largest city of Russia on Monday.

Witnesses in nearby carriages told Xinhua that they heard a tremendous thud before realizing there was an explosion.

"I was right in a carriage near the front. The train came to a halt after a big bang. I got off the train, only to find the door in the rear carriage had been deformed by the explosion," a young man named Pavel told Xinhua, "I went over and saw wounded passengers, and then I started helping them along with others."

Photos and videos published on social media show that the explosion site was covered in a heavy smoke with debris scattering around, and the wounded were lying on the platform waiting for further rescue after first aid treatment.

According to Xinhua reporters, people kept heading to the Sennaya Ploshchad (Square) station after the explosion, where a security cordon surrounding the station was set up by the police. Ambulances, fire engines and helicopters were mobilized to assist the on-site rescue.

All subway stations were closed at 3:40 p.m. local time (12:40 GMT) in St. Petersburg after evacuating all passengers.

At least 10 death tolls and 50 injuries were previously reported by Russian media.

No Chinese citizens have been reported in the casualties so far. The Chinese embassy and consulate in Russia are further verifying relevant information.

In addition, another explosive device was reportedly found at the metro station "Ploshchad Vosstaniya", which did not explode.

St. Petersburg authorities announced three days of mourning in the city while Russian President Vladimir Putin, who was holding a meeting outside St. Petersburg, expressed condolences to the injured and families of the deceased.

Local authorities have decided to provide 23 million rubles (about 408,000 U.S. Dollars) for assistance to the families of those killed and injured, Interfax news agency quoted the press service for Deputy Governor Anna Mityanina as saying.

Russia's Investigative Committee (IC) has opened a criminal case on the explosion.

"Despite the fact that the criminal case has been launched under Article 205 of the Russian Criminal Code (terrorist act), the investigators will probe other potential causes of the incident," IC's spokeswoman Svetlana Petrenko said.

According to an official statement by the IC, a group of experienced investigators and criminologists from the central office of the Investigative Committee was sent to St. Petersburg. Initial investigative actions are being carried out.

The prosecutor's office in St. Petersburg has also begun an investigation in connection with the explosion. The prosecutor of the city Sergei Litvinenko has already arrived at the scene to coordinate the actions of law enforcement agencies, according to a statement from the Prosecutor General's of Russia Office.

"We will do everything we can to clarify all the points that contributed to the commission of the terrorist act, so that this does not happen again in the future," TASS quoted Representative of the Prosecutor General's office Alexander Kurennoy as saying.

Earlier in the day, Putin said that Russian law enforcement agencies are working to determine circumstances of the explosion, and all possible causes are being considered, including terrorism.

ST. PETERSBURG, Russia, April 3 (Xinhua) -- Russian Health Minister Veronika Skvortsova said at least 10 people had been killed and 37 others were injured in an explosion of an unknown explosive device with destructive elements in St. Petersburg.

The explosion took place around 14:40 in a train carriage on the runway between the metro stations Technological Institute and Sennaya Ploshchad (Square) in Line 2 in St. Petersburg, the second largest city of Russia on Monday.

Witnesses in nearby carriages told Xinhua that they heard a tremendous thud before realizing there was an explosion.

"I was right in a carriage near the front. The train came to a halt after a big bang. I got off the train, only to find the door in the rear carriage had been deformed by the explosion," a young man named Pavel told Xinhua, "I went over and saw wounded passengers, and then I started helping them along with others."

Photos and videos published on social media show that the explosion site was covered in a heavy smoke with debris scattering around, and the wounded were lying on the platform waiting for further rescue after first aid treatment.

According to Xinhua reporters, people kept heading to the Sennaya Ploshchad (Square) station after the explosion, where a security cordon surrounding the station was set up by the police. Ambulances, fire engines and helicopters were mobilized to assist the on-site rescue.

All subway stations were closed at 3:40 p.m. local time (12:40 GMT) in St. Petersburg after evacuating all passengers.

At least 10 death tolls and 50 injuries were previously reported by Russian media.

No Chinese citizens have been reported in the casualties so far. The Chinese embassy and consulate in Russia are further verifying relevant information.

In addition, another explosive device was reportedly found at the metro station "Ploshchad Vosstaniya", which did not explode.

St. Petersburg authorities announced three days of mourning in the city while Russian President Vladimir Putin, who was holding a meeting outside St. Petersburg, expressed condolences to the injured and families of the deceased.

Local authorities have decided to provide 23 million rubles (about 408,000 U.S. Dollars) for assistance to the families of those killed and injured, Interfax news agency quoted the press service for Deputy Governor Anna Mityanina as saying.

Russia's Investigative Committee (IC) has opened a criminal case on the explosion.

"Despite the fact that the criminal case has been launched under Article 205 of the Russian Criminal Code (terrorist act), the investigators will probe other potential causes of the incident," IC's spokeswoman Svetlana Petrenko said.

According to an official statement by the IC, a group of experienced investigators and criminologists from the central office of the Investigative Committee was sent to St. Petersburg. Initial investigative actions are being carried out.

The prosecutor's office in St. Petersburg has also begun an investigation in connection with the explosion. The prosecutor of the city Sergei Litvinenko has already arrived at the scene to coordinate the actions of law enforcement agencies, according to a statement from the Prosecutor General's of Russia Office.

"We will do everything we can to clarify all the points that contributed to the commission of the terrorist act, so that this does not happen again in the future," TASS quoted Representative of the Prosecutor General's office Alexander Kurennoy as saying.

Earlier in the day, Putin said that Russian law enforcement agencies are working to determine circumstances of the explosion, and all possible causes are being considered, including terrorism.