Clashes have broken out between police and protesters in Athens as the city marks the first anniversary of the fatal police shooting of a teenager.

Riot police fired tear gas at demonstrators as they marched. People were throwing rocks at police, and rubbish bins were set on fire.

The protests are nothing like the riotous scenes of last year, a BBC correspondent says.

But aggressive tactics have not endeared police to protesters, he adds.

Six thousand police were deployed on the streets of Athens.

In Thessaloniki, Greece's second-largest city, demonstrators threw petrol bombs at police and smashed the storefront of a Starbucks cafe.

To head off trouble, riot police carried out a series of raids on Saturday across Athens, arresting more than 150 people, reports said.

The BBC's Malcolm Brabant in Athens says they have been forming snatch squads to deal with breakaway groups apparently intent on causing damage.

Greece's government has warned it will have a zero tolerance policy towards violence.

Family and friends of teenager Alexandros Grigoropoulos will hold a memorial service in Exarchia district of Athens on Sunday evening to mark a year since his killing.

They have appealed for calm, but posters have appeared in the capital saying: "We won't forget, we won't forgive."

On Saturday, two cars were set alight in the Exarchia.

Police said up to 150 foreign anarchists arrived this weekend from Italy, France and other European countries.

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