Youths hurled rocks at banks and shop windows in central Athens and riot police retaliated by firing tear gas during protest marches on Monday to mark the second anniversary of the police shooting of a teenager which sparked days of mass rioting.
It also coincides with fears of a possible new terrorist attack following the arrest of six terrorists over the weekend.
Protesting youths could be seen pelting police and the finance ministry building with rocks as well as smashing shops, banks and phone booths.
Authorities had already put in place precautions with police closing roads and deploying more than 5,000 officers along the streets of the capital, Athens.
15-year-old Alexis Grigoropoulos was shot dead by police in December 2008, sparked massive riots in cities across the country, lasting two weeks and causing extensive damage to state buildings, businesses and cars.
More than 3,000 students marched from Athens University’s in the first of several rallies scheduled.
The country’s public servants union ADEDY marked the event with a three-hour work stoppage, while protest marches were also being held in the northern port city of Thessaloniki.
The rallies in Athens were to culminate with a vigil Monday night near the site where the teenager was killed in the central Athens district of Exarchia.
In October, a Greek court convicted a 38-year-old policeman of murder and sentenced him to life imprisonment for the shooting of the teenager.
A second officer, 32-year-old Vassileos Saraltiotis, was convicted of complicity and sentenced to 10 years in prison.
The policemen claim the teenager died from a ricocheting bullet when the officers fired a warning shot after they came under attack from the youths during a night patrol in Exarchia.
The lawyer for the two officers said they would both appeal the sentences.
The Greek capital has since been beset by violent protests and multiple bombings against police stations, banks and state agencies by leftist and anarchist groups. The violence has left one policeman dead and seven wounded, several seriously.
On Sunday, police arrested six people on terrorism charges, including two suspected members of the Conspiracy of the Cells of Fire that were allegedly behind a series of parcel bombs sent to European leaders and embassies last month.
Counter-terorism officers also seized heavy weaponry and explosives in raids in Athens and in several other cities around the country.
The six charged are expected to appear before an examining magistrate on Tuesday.
Police officials said the arrests were made over the weekend amid serious indications that a new terrorist attack was being planned.
Greece has a long history with domestic terrorism, which has left more than 40 people dead in the last three decades, including a British military attache, a CIA station chief, police officers, journalists and businessmen.