Greek police fired several rounds of teargas to disperse rioting protesters as thousands marched in Athens on Sunday to mark the police killing of a teenager, which unleashed the country's worst riots in decades last year.

Hooded youths broke from the march of more than 3,000 people to smash shop windows, set fire to garbage bins and hurl stones at police, eyewitnesses said.

"We are using teargas on several fronts where youths are damaging stores and setting fire to garbage bins," said a police official, who requested anonymity.

Greece's new socialist government has deployed more than 6,000 police on Athens streets to try and avert a repeat of last year's unrest that hit the capital and major cities causing millions of euros of damages.

The riots were fuelled by youth discontent with high unemployment and economic hardship. Protesters on Sunday said the government may have changed in October but not much had improved in their everyday lives.

"It's been a year since police murdered the boy and the government which caused the murder has collapsed but nothing has changed in terms of police brutality," said Panos Garganas, 63, a university employee. "We want more jobs, more education and no more police."

Last year, thousands of people took to the streets and clashed with police hours after the killing of the 15-year-old last year, destroying shops, attacking public buildings and burning cars, in rampages that went on for weeks.

Protesters in central Athens unfolded a banner reading "Remember, remember, the 6th of December". Some dressed in black chanted "Policemen, Pigs, Murderers".

More protests were expected on Monday. An evening memorial service was planned in the Exarchia district, where the 15-year-old was shot dead.

Greek police said about 150 foreign anarchists had arrived from Italy, France and other European countries. Police arrested 75 youths on Saturday, including 5 Italians and one Spaniard, for carrying wooden sticks or throwing stones at police.