Greek police fired tear gas and protesters hurled firebombs as more than 40,000 people took to the streets to slam the government and voice outrage at last month's train disaster that killed 57 people.
Television footage showed clashes breaking out at Syntagma Square near parliament in central Athens.
Police fired tear gas and stun grenades as demonstrators tried to surround them, hurling firebombs and rocks.
As protesters retreated, they smashed traffic lights and shop windows and set bins on fire.
The 28 February tragedy exposed decades of safety failings in Greek railways and put major pressure on the conservative government ahead of national elections.
Police said 25,000 people protested in Athens, as well as around 8,500 in each of the country's next largest cities, Thessaloniki and Patras. Brief clashes also broke out in Patras, police said.
The protests were accompanied by a 24-hour strike -- the biggest yet in days of industrial action that followed the disaster -- this time called by Greece's leading private and public sector unions.
The walkout shut down the civil service, flights and ferries.
Many protesters urged the government to resign over what was the country's deadliest rail accident.
"This crime will not be forgotten," demonstrators from the country's communist union PAME chanted as the crowd marched towards parliament.
Students shouted "murderers" and marchers threw flyers of Greek Prime Minister Kyriakos Mitsotakis wearing a stationmaster's cap, captioned "it's everyone's fault but mine".
'Things have to change'
The rail disaster occurred shortly before midnight when a passenger train crashed head-on into a freight train in central Greece after both were mistakenly left running on the same track.
Most of the passengers were students returning from a holiday weekend.
"Things have to change in this country, we simply cannot mourn all these deaths," said protester Stavroula Hatzitheodorou, "We hope that things will change in these elections."
A stationmaster and three other railway officials have been charged, but public anger had focused on long-running mismanagement of the network and the country had been rocked by a series of sometimes violent mass protests.
'We don't want their money'
Last week, some 65,000 people took part in demonstrations around the country, including around 40,000 in Athens.
In addition to the 57 people who were killed, several victims remain in hospital, including one passenger who is fighting for his life.
The Italian state-owned company operating rail services in Greece, Hellenic Train, said those hurt in the accident and the families of the dead would each between €5,000 and €42,000 "to cover immediate needs".
"This is in no way an admission of responsibility," the company said.
The father of one of the passengers who died rejected the offer.
"We don't want their money ... this was mass murder, I refuse to accept an apology from murderers," Pavlos Aslanidis told Alpha TV.
"Had this been a serious country, everybody at the transport ministry would be in handcuffs," he said.
Polls slipping
Greece's transport minister resigned after the crash and Mr Mitsotakis had sought to soothe public anger by repeatedly apologising and vowing a transparent probe.
Rail traffic ground to a complete halt across the country after the accident, although acting Transport Minister Georgios Gerapetritis said this week that services would gradually resume from 22 March.
Mr Gerapetritis said a report by experts investigating the tragedy will be delivered in a month.
Investigators have separately opened a probe into possible railway funds mismanagement over the last 15 years.
Mr Gerapetritis and former transport ministers will appear before a parliamentary committee on Monday to answer politicians' questions on the tragedy.
With public anger mounting before elections expected in May, Mr Mitsotakis has seen a lead in the polls slashed to just over 3% in recent surveys.
He had come under fire for blaming "human error" for the accident and the stationmaster on duty at the time, who allegedly routed the trains onto the same stretch of track by accident.
But railway unions had long been warning about problems on the underfunded and understaffed train network.
Mr Mitsotakis had been expected to set an April election date. Ballots are now expected in May.
