Greece's prime minister on Sunday asked for forgiveness from the families of the 57 dead in the nation's worst rail disaster as thousands of furious protesters rallied in Athens and clashed with police.

"As prime minister, I owe it to everyone, but especially to the victims' relatives, (to ask for) forgiveness," Kyriakos Mitsotakis wrote in a message addressed to the nation.

"For the Greece of 2023, two trains heading in different directions cannot run on the same line and no one notice," Mitsotakis said in the message posted on his Facebook page.

The crash between passenger and freight trains near the city of Larissa on Tuesday has sparked widespread outrage across Greece.

Thousands of angry demonstrators gathered outside the parliament in Athens on Sunday following a call by students, rail workers and public sector employees.

AFP journalists saw violent clashes erupt between police and the protesters.

They had released hundreds of black balloons into the sky in memory of the dead, with some holding signs reading "Down with killer governments", while train and metro services were paralyzed by strike action.

Michalis Hasiotis, head of the chartered accountants' union, told AFP they felt "an immense anger", blaming "the thirst for profit, the lack of measures taken for the passengers' protection" for the disaster.

Relatives and loved ones of those killed were also expected to gather Sunday for a memorial outside Larissa station, central Greece, near the site of the accident.

The station master implicated in the disaster was due in court on Sunday, a hearing postponed from the previous day, where he may face charges of negligent homicide.

- Grief and anger -

Hellenic Train, the rail company that has become the focus of some of the anger expressed in the wake of the crash, released a statement late Saturday defending its actions.

Hundreds of people had demonstrated during the week outside their Athens headquarters, and one legal source has said that investigators are looking at the possibility of bringing charges against senior members of the company.

Over the last few days, rail union officials have insisted they warned the company about the safety issues on the line. Hard questions are also being asked of the government over its failure to pursue rail safety reforms.

The demonstrations and vigils across Greece have expressed a combination of grief and anger at the disaster, which happened when a passenger train and a freight train collided.

Syntagma Square, next to the Greek parliament in Athens, was the scene of clashes between police and angry protesters on Friday night.

Candle-lit marches and ceremonies have also been held in memory of the victims, many of them students who were returning from a weekend break.

"What happened was not an accident, it was a crime," said one protester, Sophia Hatzopoulou, 23, a philosophy student in Thessaloniki.

"We can't watch all this happen and remain indifferent."

At least nine young people studying at Thessaloniki's Aristotle University were among those killed on the passenger train.

- 'New elements' in case -

The station master at Larissa, whose identity has not been made public, has admitted responsibility for the accident, which happened after the two trains ran along the same track for several kilometers.

The 59-year-old man, if he is charged with negligent homicide, faces life in jail if convicted.

But his lawyer Stefanos Pantzartsidis insisted Saturday: "In the case, there are important new elements that need to be examined."

Details have emerged in Greek media of the station master's relative inexperience in the post and the fact that he was left unsupervised during a busy holiday weekend.

- Safety warnings -

"These are particularly difficult days for the country and for our company," Hellenic Train said in a statement late Saturday, pointing out that it had lost nine of its own employees in the crash.

Its staff were quick to reach the scene of the disaster and had been working closely with rescue teams and the authorities ever since, the company added.

Kostas Genidounias, the head of the train drivers' union OSE, has said they had already warned the authorities about safety failings on the line where the crash happened.

And union leaders at Hellenic Train sounded the alarm just three weeks ago.
"We are not going to wait for the accident to happen to see those responsible shed crocodile tears," they said at the time.