Prime Kyriakos Mitsotakis on Thursday pledged "absolute transparency" in the probe into Greece's deadliest rail crash that killed 57 people, a day after mass protests rocked the country.

Speaking at the start of his first cabinet meeting since the February 28 accident, he pledged "absolute transparency in the investigation to uncover errors".

He also promised "immediate actions to improve the problematic situation in the railways", and that he would "move heaven and Earth" to ensure improvements to safety systems were completed.

The crash happened when a passenger train and a freight train collided head-on in central Greece after mistakenly being directed onto the same track.

"We are all responsible for this, we must be brave enough to admit it," said Mitsotakis -- who is fighting to be elected at polls within months -- in televised remarks.

The conservative leader said he felt "rage and shame" when he heard conversations between the local station master, who allegedly accidentally directed the trains onto the same track, and other railway staff.

But he also apologised again and said that "we... must not hide behind a series of human errors".

Mitsotakis has been accused of trying to shift the blame onto the inexperienced station master, with critics saying government mismanagement was the main reason for the accident.

On Wednesday, tens of thousands of protesters took to the streets nationwide, in the biggest demonstrations yet since the crash, with clashes erupting in Athens in Thessaloniki.