In letters to the union heads, the premier urged them to cancel rallies to avoid deterioration of the situation following days of widespread riots sparked by the fatal police shooting of a teenager Saturday night.

Karamanlis said some quarters have used the tragic shooting as an "alibi" to engage in "unprecedented violence" against innocent citizens, property and all of society, adding that such conduct caused harm to democracy.

"Demands and protest are absolutely respected and inviolable democratic rights. However, due to the situation that has developed over the last few days, I call on you to show the required responsibility and cancel the planned rallies for tomorrow's 24-hour strike, so that there is no opportunity to exploit an event by wage-earners in order to proceed with new acts of catastrophic violence," Karamanlis wrote in the letters.

The trade unions have rejected the prime minister's appeal, according to local media.

Since Saturday, gangs of youngsters have fought pitched street battles with riot police, setting fire to vehicles, shops and public buildings, and even the newly installed big Christmas tree in the capital's central Syntagma Square.