Violent clashes broke out between police and protesters outside the Greek parliament in Athens on Sunday as thousands attended a rally following the nation's worst rail disaster that killed 57.

Some demonstrators set fire to rubbish bins and threw Molotov cocktails, while police responded firing tear gas and stun grenades, clearing Syntagma Square of the protesters within a few minutes.

Police said 12,000 people had gathered by the large esplanade in front of the parliament to demand accountability for Tuesday's head-on collision near the central city of Larissa that has sparked widespread outrage.

They had released hundreds of black balloons into the sky in memory of the dead, with some holding signs reading "Down with killer governments".

Train and metro services have been paralysed by strike action. (AFP)