The largest protests were held in Thessaloniki, Greece's second-largest city, and in the capital Athens, where thousands chanted “this crime will not be forgotten” as they reached a police cordon outside a private rail operator.

Some 25,000 people were marching in Athens alone, a police spokeswoman told AFP, with many calling on Prime Minister Kyriakos Mitsotakis to resign.

Police said another 8,500 people were demonstrating in the country's second city of Thessaloniki and the same number in the western port city of Patras.

The strike also kept ferries to the Greek islands at port, left public hospitals running with emergency staff, halted public transport services and led to class cancellations at state-run schools.

"This crime will not be forgotten," demonstrators from the country's communist union PAME chanted as the crowd marched on parliament in the capital. 

Students shouted "murderers" and marchers threw flyers of Prime Minister Kyriakos Mitsotakis wearing a stationmaster's cap, captioned "it's everyone's fault but mine".

The accident occurred at around 11:30 pm on Tuesday (2130 GMT) near Larissa in the Tempe Valley of Thessaly, central Greece, about 380 kilometers (236 miles) north of Athens, on the main railway line between the capital and Thessaloniki.

The passenger train, with 342 passengers and ten employees on board, slammed head-on into a freight train traveling on the same track in the opposite direction, officials said, in what is considered the country's worst-ever rail disaster.

A fire broke out, apparently in the restaurant car that was among the three leading carriages.

Most of the casualties were in the first three carriages, which included first-class seats.

Dozens of people were hurt most of them students returning to Thessaloniki after a long Carnival weekend. While all ten train employees are believed to have died in the crash.

Thursday's 24-hour strike is the biggest yet in days of industrial action that followed the disaster, this time called by Greece's leading private as well as public sector unions.

The walkout shut down the civil service, flights and ferries.

"Things have to change in this country, we simply cannot mourn all these deaths," said Athens' protester Stavroula Hatzitheodorou, in reference to deadly wildfires that have gripped Greece in recent years as well as the train crash.