Greek police fired tear gas at firebomb-throwing protesters outside parliament Wednesday as tens of thousands demonstrated over the country's worst rail tragedy and called on the prime minister to quit.

The accident occurred on February 28 when a freight train crashed head-on with a passenger train, carrying mostly students, killing 57 people while 14 others remain in hospital. The biggest protests were in Athens, where over 40,000 people flooded the streets, waving banners that read "It's not an accident, it's a crime" and "It could have been any of us on that train".

Outside parliament, dozens of masked, black-clad youths hurled Molotov cocktails and stones at riot police, who responded by firing volleys of tear gas, an AFP reporter said. Demonstrators also torched a van a few blocks away. In Greece's second city of Thessaloniki, riot police fired tear gas at stone-throwing protesters outside the local railway station.

More than 65,000 demonstrators took to the streets nationwide to protest the train crash near the central city of Larissa, according to police. "I am here to pay tribute to the dead but also to express my anger and my frustration," Athens protester Niki Siouta, a 54-year-old civil engineer, told AFP.

"This government must go." Fifteen people were arrested in Athens for violence related to the demonstrations. Alongside the protesters, Greek civil servants staged a 24-hour walkout while doctors, teachers, and transport workers also went on strike.

"This government opts to spend money on the police and the army, but not for our safety," said Thanassis Oikonomou, a striking worker and bus union representative.

Many protesters waved signs reading "Call me when you get there", a rallying cry for demonstrators which reportedly originates from a phone call between a mother and her son shortly before he died in the crash. Protests continued into the evening, with several thousand demonstrators gathering in both Athens and Thessaloniki.

Calls are growing louder for Prime Minister Kyriakos Mitsotakis -- who is fighting for re-election this spring -- to quit over the tragedy, which has cast a harsh spotlight on decades of government mismanagement of the rail network. A stationmaster, who admitted forgetting to reroute one of the trains, has been arrested and charged, but the government has been criticised for seeking to shift the blame mainly on him.

Critics say he was inexperienced and was left working alone during a busy holiday period, and accuse officials of failing to pursue safety reforms on the network. Greece's transport minister resigned on March 1 and Mitsotakis has apologised to victims' families, pledging to get to the root of what happened, and embarked on a flurry of public appearances in an apparent bid to soothe anger.

The prime minister and other politicians suspended election campaigning in the wake of the tragedy. There is now speculation that the polls, expected in April, could be delayed until May. On Wednesday, Mitsotakis met EU officials including the executive director of the bloc's rail agency and the European Commission's director for land transport who were invited to Athens, the premier's office said.

The European officials presented some "first thoughts" on improving network safety and Mitsotakis said Greece would "fully" use their expertise, his office added. Giorgos Gerapetritis, Greece's acting transport minister, said rail passenger transport might resume by the end of March.

Speaking at a press conference, he also conceded that if safety systems had been fully automated, "the accident would not have happened". Political life will resume Thursday with a cabinet meeting after a period of national mourning, but the premier seems in no rush to confront the issue of the looming polls.