Police said between 100 and 150 anarchist protesters had started throwing firebombs at riot police and setting trash cans on fire in the central Athens neighborhood of Exarcheia.
This is far from the place where celebrations are taking place by supporters of the "no" vote in Sunday's referendum.
There was no indication that the violence was connected to the political campaigns involved in the vote.
The results of Sunday’s referendum — 61 percent voted "no," compared with 39 percent for "yes" — left the country’s future in the European Union and its euro currency uncertain.
A total of 6.16 million Greeks voted in Sunday's referendum, or 62.5 percent of eligible voters. The poll needed a minimum 40 percent turnout to be valid.
