Greek riot police have formed a cordon around parliament, in a face-off with thousands of people protesting the death of a teenager from police gunfire.

About 7,000 protesters marched Thursday through central Athens, marking the 13th day of protests triggered by the shooting of a 15-year-old boy.

Flames engulf a policeman from a petrol bomb thrown by rioters outside parliament in Athens, 18 Dec 2008

Hundreds later broke off from the rally and began hurling rocks and firebombs at police, who quickly retaliated with tear gas to disperse the crowd.  

Other demonstrators set fire to trash cans, cars and attempted to burn down the city's Christmas tree.

About 300 protesters also marched through the northern city of Salonika.

Earlier in the day, labor unions and teachers staged work stoppages to express their growing dissatisfaction with the social and economic reforms of the government of Prime Minister Costas Karamanlis.

Police said a 16-year-old was shot in the hand late Wednesday, but said they were not in the area at the time of the incident and do not know who fired the bullet.

Retailers have reported more than a billion dollars in damage and lost sales in the violence.

Authorities said rioting triggered by the shooting death is the worst in the country in decades.

Greek media have described the protesters as self-styled anarchists and youth belonging to far-left militant groups. 

Some information for this report was provided by AFP, AP and Reuters.