Greek Prime Minister Kyriakos Mitsotakis has asked the country's Supreme Court to give "top priority" to the criminal cases triggered by last week's fatal train disaster, his office said Monday.

"The Greek people want an immediate and thorough clarification of the criminal incidents related to this tragic accident," Mitsotakis wrote in a letter to the court's prosecutor about the collision, which killed at least 57 people and stoked public anger.

The letter specified that the court investigation into the tragedy was separate from the one already launched by government-appointed experts.

On Sunday, Mitsotakis, who is expected to seek re-election in April, asked for forgiveness from the families of those killed in Greece's worst rail disaster as thousands of furious protesters rallied in Athens and clashed with police.

The crash occurred last Tuesday when a freight train collided head-on with a passenger train carrying over 350 passengers, many of them young students.