Police clashed with hooded youths and the rector of Athens university was injured Sunday as violence flared in demonstrations marking the anniversary of a teenager's killing by a policeman last year.

Athens University rector Christos Kittas was rushed to hospital after protesters broke into the university's central Athens offices to occupy them, a police source said. Media reports said his injuries were light.

In the northern city of Thessaloniki, a Starbucks branch was set on fire and clashes also broke out between protesters and riot police on the sidelines of another large demonstration in 15-year-old Alexis Grigoropoulos' memory.

Around 40 people were detained during the Athens protest, during which a number of shops and banks had their windows smashed.

Police had hoped to avoid a repeat of riots that followed the death of Grigoropoulos last year and caused millions of euros in damages, making dozens of arrests on the eve of the Sunday protest.

Thousands of protesters marched in Athens on Sunday to mark the anniversary.

The demonstration, one of a series planned in the capital and main Greek cities, followed dozens of arrests on Saturday night in a raid on a suspected anarchist hideout and mop-up operations in the capital.

Over 6,000 police were on duty in Athens alone. Another 3,000 were mobilised in Thessaloniki, local police said.

A family memorial service for the teenager was held early Sunday in the cemetery of Palio Faliro. His mother appealed for demonstrations in his memory to remain peaceful, media reports said.

Greece's recently-elected socialist government, mindful that the 2008 riots that caused millions of euros in damages badly discredited its conservative predecessors, had warned protesters against resorting to violence.

On Saturday, police carried out a series of raids in Athens and detained more than 130 people after two cars were set on fire in the central district of Exarchia, where Grigoropoulos was gunned down last year.

Twelve people, including five Italians and three Albanians, were arrested over the torching of the cars and another 41 people were arrested in the western district of Keratsini after briefly occupying the local town hall.

In a separate raid in the same area, police arrested a further 22 people in what they said was an anarchist hideout. Officers found two petrol cannisters, sledgehammers and 13 gas masks on the premises, police said

"The search confirmed prior information that this location was used to create explosives and launch attacks," a police statement said.

Around 500 people took part in protest marches in the northern city of Thessaloniki on Saturday night, local police said. Ten people were arrested after the demonstration, though police said two were for reasons unrelated to the demonstrations.

Another demonstration by students and school pupils will be held on Monday.

Students have occupied dozens of universities and schools to mark the teenager's killing, according to staff unions.

Grigoropoulos was shot dead by a police officer who claimed he fired into the air whilst under attack from youths.

A few dozen foreign demonstrators are also believed to have travelled to Greece for the commemoration, a police source said.

The policeman accused of the teenager's death is due to go on trial on January 20 charged with homicide.

The trial was originally scheduled to be held this month but judicial authorities postponed it and relocated the proceedings to Amfissa, a town some 185 kilometres (115 miles) northwest of Athens.