The two trains - a passenger train travelling from Athens to the northern city of Thessaloniki, and a cargo train from Thessaloniki to Larissa, collided head-on outside the city of Larissa in central Greece, Konstantinos Agorastos, governor of the broader Thessaly region, told SKAI TV.
He added that the first four carriages had derailed, while the first two were "almost completely destroyed. Agorastos said about 250 passengers were evacuated safely to Thessaloniki on buses.
Footage from the scene showed derailed carriages, badly damaged with broken windows and thick plumes of smoke, as well as debris strewn across the road.
"There was panic in the carriage, people were screaming," a young man who was evacuated to a nearby bridge told SKAI TV.
"It was like an earthquake," Angelos Tsiamouras, another passenger, told ERT.
In the early hours of Wednesday, rescue crews were still searching for survivors.
"The evacuation of passengers is underway in very difficult conditions given the severity of the collision of the two trains," fire brigade spokesperson Vassilis Varthakogiannis said in a televised address.
Local media reported about 350 people were travelling on the train.
Reuters