ATHENS - Protests held in several Greek cities on Thursday in the memory of Alexandros Grigoropoulos, a teenager shot dead by a police officer in Athens in 2008, were marred with violent incidents.

At least 27 demonstrators in the Greek capital, Thessaloniki in northern Greece and Chania on Crete island were detained and held in custody for participating in clashes with anti-riot police forces and vandalism, according to Greek national news agency AMNA.

According to police sources, among the detained were minors.

No injuries have been reported, but material damages.

In Athens, the focus of the scuffles on Thursday evening was at the district of Exarchia, where 15-year-old high school student Grigoropoulos was shot dead on Dec. 6, 2008, by a policeman, who was later sentenced to life imprisonment.

Following a march organized by Leftist parties and anti-establishment groups, dozens of hooded youth hurled petrol bombs and pieces of marble cut down from pavements, put rubbish bins on fire and set up road blocks.

Police responded by firing tear gas and stun grenades and deployed water cannons to disperse the rioters.

Approximately 5,000 police officers had been deployed, while a police helicopter and drones were flying throughout the day over the city center of Athens to monitor the customary rallies.

Similar clashes broke out on Thursday evening also in Thessaloniki and Chania.

Earlier in the day, demonstrations held by students' unions in central Athens and Thessaloniki had also ended in violence.

Grigoropoulos' death ten years ago was followed by a string of rioting across Greece for two weeks in protest of police violence. The anniversary has been marred by violent incidents every year.

The victim's mother, Gina Tsalikian, has repeatedly made pleas in recent years against violence.

"If my child was alive, he would speak against such clashes. He would not like to see his name linked to violence and vandalism. These incidents sully Alexandros' memory," Tsalikian told local ANT1 television channel.

"Security is not safeguarded with weapons aimed at minors, but with democracy, justice, transparency and respect of citizens' rights," the family's lawyers added in a press release on Thursday.