TIMELINE: How the Greek crisis unfolded

(Reuters) - Riot police fought running battles with hundreds of protesters outside Greece's parliament on Tuesday, in a fourth day of anti-government clashes triggered by the police shooting of a teenage boy.

Here is a timeline of events.

DEC. 6:

-- A police officer shoots dead 15-year-old Alexandros Grigoropoulos in central Athens. The policeman says he fired warning shots but witnesses say he aimed at the boy after a verbal altercation with some youths in the Exarchia district.

-- Thousands protest in Athens hours later, throwing petrol bombs, smashing shop windows and setting fires. Police clash with rioters using teargas, choking the capital's center.

-- Riots spread across Greece, to the northern cities of Thessaloniki, Komotini and Ioannina as well as the tourist resorts of Crete and Corfu. Police arrest six people for looting smashed shops and one is held for gun possession.

-- Interior Minister Prokopis Pavlopoulos makes a public apology and later submits his resignation, which is rejected by Prime Minister Costas Karamanlis.

-- Riots sweep across Greece through the night and two police officers are arrested over the teenager's death.

DEC. 7:

-- Thousands of demonstrators chanting "Killers in Uniform" march for a second day in Athens, Thessaloniki, Crete, Patras, Corfu and Ioannina.

-- Karamanlis sends a letter to the dead teenager's family, expressing his sorrow and promising the culprits will be punished. Pavlopoulos appeals for calm.

-- One police officer is charged with murder and another with abetting him.

-- Protesters in Athens and Thessaloniki clash with police and hurl fire bombs along the cities' main streets setting shops, buildings and cars alight.

-- The mayor of Athens postpones the launch of Christmas festivities.

DEC. 8:

-- Demonstrators launch a third day of protests and occupy university buildings. Dozens of marches bring Athens to a standstill as shop owners count the damage.

-- Hundreds battle with police in more than 10 cities across the country, including Thessaloniki.

-- Protests spread outside Greece, when 15 demonstrators occupy the Greek consulate in Berlin in solidarity.

-- Thousands of demonstrators march in the evening through the heart of Athens, damaging and looting scores of shops, destroying banks and attacking ministries. The city's huge Christmas tree goes up in flames.

-- Karamanlis holds government meeting and Pavlopoulos says there will be no tolerance for such events.

DEC. 9:

-- Police have arrested some 200 people, some for looting during the protests.

-- The opposition socialist party calls for elections to end the four days of protests.

-- Karamanlis meets President Karolos Papoulias and party political leaders to find ways to end the rioting which has threatened the government.

-- The funeral of Grigoropoulos takes place in the outskirts of Athens.

-- Karamanlis appeals to unions to unite against the riots and cancel a protest rally during a 24-hour strike scheduled for Wednesday. Both requests are rejected.