At least 36 people have been killed and dozens injured after two trains crashed head-on at high speed in Greece.

A passenger train travelling from Athens to the northern city of Thessaloniki collided with a freight train outside Larissa just before midnight.

Police said the 59-year-old stationmaster in Larissa was arrested, and a further two people detained for questioning.

The collision occurred as the passenger train emerged from a tunnel near the Vale of Tempe, a gorge that separates the regions of Thessaly and Macedonia.

Police said several carriages came off the rails, with at least three of them catching fire.

Many of the victims were thought to be university students.

Two carriages of train in ‘nightmare’ crash ‘no longer exist’ – follow live updates

Regional governor of Thessaly, Kostas Agorastos, said 250 survivors had been evacuated to Thessaloniki on buses.

The collision was so violent that the first two carriages of the passenger train “no longer exist”, he told Skai television.

“They were travelling at great speed and one (driver) didn’t know the other was coming,” he said.

Survivors said several passengers were thrown through the windows due to the impact. Some victims were found up to 40 metres away.

A passenger who escaped from the fifth carriage told the television network: “Windows were being smashed and people were screaming…”

Rail operator Hellenic Train expressed “its deep sorrow for the tragic accident” and said the passenger train had about 350 people on board.

Officials said this included university students returning home after a long holiday weekend marking the beginning of Greek Orthodox lent.

“This is a terrible tragedy that is hard to comprehend,” said deputy health minister Mina Gaga. “I feel so sorry for the parents of these kids.”

The government declared three days of national mourning until Friday, with flags flying at half-mast in a tribute to the victims. Greek Prime Minister Kyriakos Mitsotakis was to visit the scene later in the day.

Greek President Katerina Sakellaropoulou said she was cutting short a visit to Moldova to return to Greece.

“This is an indescribable tragedy,” said government spokesman Giannis Oikonomou.

Footage showed rescuers wearing head lamps surrounded by thick smoke, pulling pieces of mangled metal from the carriages to search for trapped people.

Others scoured the field with flashlights and checked underneath the wreckage.

After sunrise, rescuers turned to heavy machinery to start moving large pieces of the trains.

Several of the dead are believed to have been found in the restaurant area near the front of the passenger train.

“This is a terrible night,” Mr Agorastos had earlier told state-run television. “It’s hard to describe the scene.”

The regional governor added: “The front section of the train was smashed… we’re getting cranes to come in and special lifting equipment to clear the debris and lift the rail cars. There’s debris flung all around the crash site.”

One passenger told state broadcaster ERT: “There was fire next to us. We found a hole and from there we managed to get out. The wagon started to spin and then it ended up on its side and we got out.

“It was a nightmarish 10 seconds, in the flames. There was panic in the carriage, you couldn’t see around you because of the smoke.”

Stergios Minenis, a 28-year-old passenger who jumped to safety from the wreckage, said: “We heard a big bang.

“We were turning over in the wagon until we fell on our sides… then there was panic, cables (everywhere)… the fire was immediate, as we were turning over we were being burned.”

Fire service officials said 36 people were killed and at least 85 were injured. Around 60 had been taken to hospital, with six of them in intensive care.

“The evacuation process is ongoing and is being carried out under very difficult conditions due to the severity of the collision between the two trains,” fire brigade spokesperson Vassilis Vathrakogiannis told a news conference.

“We are living through a tragedy. We are pulling out people alive, injured… there are dead.”

Eight rail employees were among those killed, 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.

Ambulances from several nearby towns were called in to take injured passengers to hospitals, while rescue workers continued to work in the thick smoke.

Government officials said the army has been asked to help with the rescue.

“There were many big pieces of steel,” said Vassilis Polyzos, a local resident who was one of the first people on the scene.

“The trains were completely destroyed, both passenger and freight trains.”

He said dazed and disoriented people were escaping out of the train’s rear carriages as he arrived.

“People, naturally, were scared – very scared,” he said. “They were looking around, searching; they didn’t know where they were.”

The cause of the collision is not yet known and is being investigated.