Hundreds of people line up to donate blood in Larissa, Greece, after a train crash that killed at least 42 people and injured dozens.

The initiative has been organised by the Hellenic Red Cross.

The two trains collided near a tunnel outside the central Greek city of Larissa before midnight Tuesday.

Two carriages were crushed and a third caught fire, trapping people inside.

The head-on collision blamed on "human error" that has sparked angry protests.

A fire department spokeswoman told AFP that rescue crews had worked all night in search for survivors, but chances were dwindling.

"Time is not on our side," she said.

Prime Minister Kyriakos Mitsotakis said it was a "terrible train accident without precedent" in Greece, promising that the tragedy would be "fully" investigated.

"Everything shows that the drama was, sadly, mainly due to a tragic human error," Mitsotakis said Wednesday after visiting the disaster site.

Passengers have described scenes of horror and chaos from the crash, many dodging smashed glass and debris as the train keeled over, and breaking windows to climb out.

The train's restaurant car erupted in flames after the collision, with temperatures inside reaching 1,300 degrees Celsius, the fire department said.

For hours after the crash it was not immediately clear how many people were on board, complicating efforts to determine how many are missing.

Roubini Leontari, the chief coroner at Larissa's general hospital, on Thursday told state TV ERT that over 10 people are missing, including two Cyprus nationals.

TV footage from the wreck site Thursday showed a crane gingerly lifting the mangled remains of a train carriage, under which a body is believed to be trapped.

"It was a student train, full of kids...in their 20s," Costas Bargiotas, a senior orthopaedic doctor at Larissa General Hospital, told Skai TV.

"It was truly shocking... the carriages crumpled like paper," he said.

Rescuers at the scene said they had never dealt with a disaster of this magnitude before.

Many bodies were charred beyond recognition and some passengers were being identified from body parts.

Seventeen biological samples have been collected from remains, and from 23 relatives seeking a match, the police said.