A man has been arrested over the collision of a passenger and a freight train in central Greece that has killed at least 36 people and injured dozens, the government and police sources say.

The 59-year-old station master of a train station in the city of Larissa testified before a prosecutor and was arrested, a government official said.

A police official said the prosecutor laid misdemeanour charges against him.

He has been charged with mass deaths through negligence and causing grievous bodily harm through negligence, the official added.

The man has denied any wrongdoing and has attributed the accident to a possible technical failure, the police official said.

According to a police statement, another two two people have been detained for questioning. The cause of the collision was not immediately clear.

Transport Minister Kostas Karamanlis resigned Wednesday, saying he felt it was his "duty" to step down "as a basic indication of respect for the memory of the people who died so unfairly."

After sunrise, rescuers turned to heavy machinery to start moving large pieces of the trains, revealing more bodies and dismembered remains. Officials said the army had been contacted to assist.

Visiting the accident scene, Greek Prime Minister Kyriakos Mitsotakis said that the government must help the injured recover and identify the dead.

Costas Agorastos, the regional governor of the Thessaly area, told Greece's Skai Television the two trains collided head on at high speed.

The trains crashed just before the Vale of Tempe, a gorge that separates the regions of Thessaly and Macedonia.

Survivors said the impact threw several passengers through the windows of train cars.

They said others fought to free themselves after the passenger train buckled, slamming into a field near the gorge, about 380 kilometres north of Athens.

Carriage reached 1,300 degrees Celsius

Authorities are expecting the death toll to climb in the coming days, as temperatures in the first carriage were extremely high, fire brigade spokesman Vassilis Varthakogiannis said on Wednesday.

"It is worth noting that in the specific carriage a fire broke out and temperatures were particularly high, reaching even 1300 degrees Celsius," he told a briefing.

"The confirmed number of dead is 36 but based on these facts, and the findings from the scene of the tragedy, the number is expected to be greater."

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Eight rail employees were among those killed in the crash, including the two drivers of the freight train and the two drivers of the passenger train, according to Greek Railroad Workers Union President Yannis Nitsas.

Greece's firefighting service said some 66 people were hospitalised, including six in intensive care.

Before dawn the next day, rescuers searched through twisted, smoking wreckage for survivors. What appeared to be the third carriage lay atop the clumped remains of the first two.

Multiple cars derailed and at least three burst into flames after the two trains ran into each other at high speed just before midnight on Tuesday.

Many of the approximately 350 people aboard the passenger train were students returning from Greece's raucous Carnival, officials said.

This year was the first time the three-day festival, which precedes Lent, was celebrated in full since the start of the pandemic in 2020.

On Wednesday, the government declared three days of national mourning.

"This is an indescribable tragedy," government spokesman Giannis Oikonomou said, adding that 500 workers from emergency services were at the scene of the crash.

Reuters/AP