At least two people were killed and up to 100 injured when a passenger train carrying 800 people collided with a truck, derailing eight carriages, Russian media reported.
Dramatic images show how the carriages were left twisted and battered next to the train tracks after the collision.
The train was travelling from Kazan in Tatarstan to Adler on the Black Sea when it derailed in the southern Volgograd region near the Kotelnikovo station, some 1,200 kilometres (750 miles) south of Moscow, the Emergencies Ministry said.
“Paramedics are on site,” the ministry said, adding that 324 emergency workers were involved in the operation. Emergency helicopters have also been sent, it added.
Passengers were trapped inside the carriages, while those who managed to escape tried to free them before emergency services arrived. Some were trapped underneath the train.
Passengers were trapped inside the carriages while those who managed to escape tried to free them.
Dramatic images show how the carriages were left twisted and battered next to the railway line after the collision.
Shocked onlookers gathered around the train as people rushed to help those trapped inside.
The destroyed train car is seen after the collision.
Telegram channel Mash reported that at least two people were killed, but the death toll could rise. The Interfax news agency said up to 100 people could be injured.
An unverified video on Telegram showed at least four derailed carriages, some twisted, with people exiting one that was lying on its side next to the rails.
Mash posted a video showing the shattered remains of a Kamaz truck cabin next to the derailed train.
TASS cited unnamed Russian police sources as saying that 20 people were injured when the train collided with a Kamaz truck.
Other media said 37 people, including six children, were initially injured in the train crash, with four of them hospitalized.
Passengers at the scene posted harrowing images of overturned carriages.
The train was running from Kazan to Adler, part of the Black Sea resort of Sochi, a 47-hour journey.
The truck driver, Aslambek Usmatov, 43, was seriously injured and taken to hospital with serious injuries.
The footage shows him injured and confused after the crash, near the remains of his pickup truck’s cab, which was 50 feet from the rest of his vehicle.
About 324 Emergencies Ministry specialists and 109 pieces of equipment are working at the site of the passenger train derailment in Kotelnikovo.
Two Mi-8 emergency helicopters were dispatched to the scene.
A criminal investigation has been launched.
“All the circumstances of the incident are being established,” said Interior Ministry spokeswoman Irina Volk.
One female witness said: “This is an absolute nightmare.”
‘A KAMAZ truck flew over the railway track.
“People are trying to get out, it’s a nightmare.”
A second woman said: ‘Look, what a nightmare.
‘This is our railway junction at Kotelnikovo.
‘The carriages have collapsed, look.
“People are leaving, how horrible.”
A train passenger said: ‘A KAMAZ was about 400 metres away as we approached the crossing.
‘The crew used the emergency brake at a speed of 95 km/h (60 mph).
‘There was a loud bang.
‘All we could see was a cloud of dust.
‘I saw five cars: one upside down, one sideways and three at 45 degrees.’
He said the train driver and his assistant were alive.
Last month, two million people were killed and 70 injured in an accident involving a tourist train travelling from the desolate Arctic mining town of Vorkuta to the Black Sea resort of Novorossiysk.