At least 32 people were killed when a freight train collided with a passenger train in central Greece on Tuesday night, according to the fire brigade, with people still believed to be trapped under the debris.
Fifty-three people were seriously injured and brought to hospital, a fire brigade spokesperson said on national television, adding that the search and rescue operation continued.
Rescue workers were using cranes and other heavy equipment to try and lift the derailed train cars to search for survivors, according to reporters at the crash site near the city of Larissa.
The cause of the accident was initially unclear.
A passenger train coming from Athens en route to the northern port city of Thessaloniki collided head-on with a commercial train travelling in the opposite direction, according to initial reports from railway officials.
The passenger train, the Inter City 62, had departed from the Greek capital at 7:22 pm (1722 GMT) on Tuesday evening.
Some 350 passengers were said to have been on board the train.
Videos broadcast on local television showed several wrecked train cars at the crash site near the municipality of Tempi.
Firefighters and rescue teams were searching the debris for survivors, according to reporters at the scene.
A fire broke out in the passenger train following the collision, a survivor told state broadcaster ERT. "There was chaos and incredible noise," he added.
The crash occurred on a line connecting Athens with Thessaloniki that was modernized over the past years.
The railway official responsible for the line was arrested following the crash, ERT reported.
Despite the modernization, which included new tunnels and bridges as well as two tracks along the 500-kilometre route, there were still significant problems with the electric coordination of traffic control, according to the Greek train drivers' union.
"We travel from one part of the line to the next by radio, just like in the old days. The station managers give us the green light," the union's president Kostas Genidounias explained on state radio.
Greece's railway, Hellenic Train, is operated by Italy's state-owned railway company Ferrovie dello Stato Italiane.