ATHENS, GREECE - Riot police clashed with stone-throwing youths in Athens on Monday as violence marred a second day of demonstrations held to mark the fatal shooting of a teenager by police a year ago.

Police charged the crowd with tear gas and made 21 arrests after scores of youths, some as young as 12, hurled stones at store windows and the security forces as some 5,000 demonstrators set off towards parliament.

?The message given is that Athens (and) other major cities are not defenceless,? government spokesman George Petalotis told reporters.

Two of the youths were arrested earlier for pelting a police station with stones as thousands of students rallied to pay tribute to Alexis Grigoropoulos, who was shot dead at age 15 last year, a police source said.

Two other police stations in the suburbs of Agia Paraskevi and Kallithea were also pelted with debris, police said.

Around 5,000 people joined another demonstration in the northern city of Thessaloniki which ran its course without major incident.

Some 6,000 officers were deployed to prevent further trouble in the Greek capital after demonstrations around the country turned violent at the weekend, with at least 30 people injured and dozens arrested in Athens and other cities.

After the Monday protest disbanded, police formed a massive cordon around the University of Athens building - which was seized by protesters on Sunday - to prevent another occupation.

Around 300 young people took over buildings at Athens Polytechnic, making sorties to throw stones and Molotov cocktails at police massed outside.

Greek law heavily restricts police powers to intervene on campuses, effectively making them a safe haven for protesters.

Cars and dustbins were set alight outside the Polytechnic as the youths continued their protest.

By early evening the protesters had left the building and traffic - which had been suspended in surrounding streets - was flowing again.

The rector of Athens University, Christos Kittas, was among those injured Sunday as dozens of hooded youths broke into the building on the sidelines of a large demonstration in Grigoropoulos? memory in the city centre.

Police said 26 officers and four protesters were hurt in Sunday?s clashes but reporters at the scene of the violence said the total number injured could be higher.

Some protesters who did not take part in the clashes accused police of excessive force.

Kittas, who was hospitalised for head injuries and an irregular heartbeat, remained in intensive care Monday but his condition has improved, the director of Athens? Ippokratio Hospital told state television NET.

Another 125 people were arrested over the weekend after protests in Athens and the cities of Thessaloniki, Patras, Rhodes and Heraklion, a police source said, and 46 face charges.

Many of those arrested are foreign nationals, including five Italians, four Albanians, a Pole, a Canadian, a Turk, a Bulgarian, a Spaniard and a French national.

The police on Saturday raided a Greek anarchist club which police said was used to manufacture explosives. Two of the youths arrested at the club are the children of a former socialist minister.

Windows were smashed and several cars were damaged on Sunday in more than two dozen stores and banks in Athens, Thessaloniki and other cities, although the damage was limited compared to the riots that gripped the country a year ago.

Ten people, including the five Italians, are to face misdemeanour charges on December 16.

Students have occupied scores of university faculties and schools to mark the teenager?s killing, according to staff unions.

Grigoropoulos was shot dead on December 6, 2008, by a police officer who claimed to have fired into the air whilst under attack from youths. His parents had appealed for demonstrations to remain peaceful, media reports said.

A family memorial service for the teenager was held early Sunday in the cemetery of Palio Faliro. The policeman accused of his death is due to go on trial on January 20 charged with homicide.

The trial was originally scheduled for this month but judicial authorities postponed it and relocated the proceedings to a town northwest of Athens.