According to the Greek Fire Service, at least 29 people were killed and over 85 others were injured when a passenger train carrying over 350 people collided with a freight train on Tuesday night, just before midnight, in Tempi, central Greece, close to the city of Larissa.
A desperate search for anyone still alive has been the focus of rescuers.
"We just heard a bang... the (train) car started spinning, before ending up sideways when we managed to exit," one male passenger told Greek public broadcaster ERT.
The trains had been making their way from Athens, the country's capital, to Thessaloniki, a city in northern Greece known for its many festivals and rich cultural scene. The cause of the crash, according to the authorities, remains unclear.
The Greek Fire Service has announced that recovery activities have begun and that the death toll is likely to grow.
Photos broadcast on ERT, Greece's state-owned national TV, showed long lines of rescue vehicles alongside overturned carriages emitting heavy plumes of smoke.
Meanwhile, search teams used torches to look in carriages for any signs of life.
Greek Fire Service official Vassilis Varthakogiannis said on television that the train was carrying 350 people. According to Varthakogiannis, the rescue operation involves at least 150 firefighters, 17 vehicles, and 40 ambulances.
According to a press release issued by Hellenic Train, a Greek railroad business, "a head-on collision between two trains occurred, involving a freight train and train IC 62 which had departed from Athens to Thessaloniki."
"I wasn't hurt but I was stained with blood from other people who were hurt near me," a passenger interviewed by Protothema newspaper said.
Local media stated that relatives of passengers had gathered in Thessaloniki to witness their loved ones alight from the buses.
By 7:30 p.m. (local time), the passenger train left Athens. The incident was reported to the fire department at about 11:59 p.m. on Tuesday, according to their report.
Ferrovie dello Stato Italiane, the company in charge of running the train, claims on its website to be Greece's primary rail service, carrying 342 passengers and commercial goods every day.