Speaking to Al Jazeera, Leonidas Gotzes, head of international relations at the New York College in Athens, said there were both internal and external factors in explaining the continuing violence.
Gotzes said: "There seems to be a lot more [happening] than meets the eye. There are those that believe that there are forces working both from within and from without.
"Specifically, from within there are opposition groups, anarchists, the radical left, that seem to have jumped on the bandwagon here and are trying to destabilise the government.
"The government has been in power since March 2004. It was elected to cleanse the corruption that has accumulated over 20 years of socialist rule. This the government has had a very difficult time bringing about.
"From without, the external factors have to do with primarily the fact that in the last five years the government of Costas Karamanlis seems to have followed a foreign policy... that has upset its [European Union] allies.
"Not recognising the independence of Kosovo, preventing Macedonia from entering the Nato alliance and not supporting Cyprus' unification plan. And it is therefore not receiving political support from Europe."
Rallies by unions, students and teachers also took place in the northern city of Thessaloniki and on the island of Crete.
|The protests have caused hundreds of millions of dollars of damage [AFP]
Stathis Anestis, a spokesman for the GSEE private sector union federation which took part in the rallies, said: "We are not finished just because it's Christmas. We will continue and intensify our struggle in the next year."
A three-hour work stoppage by public workers halted all but emergency flights between 1000 and 1300 GMT on Thursday.
Transport services were frozen, while doctors and teachers walked off the job, reviving memories of last week's 24-hour national strike.
Adding to tensions in the capital, before further protests planned later on Thursday and Friday, police said a 16-year-old had been shot in the hand by an unidentified gunman in Athens late on Wednesday.
"Not only policeman carry guns," said Panagiotis Stathis, a police spokesman, strongly denying any officers were involved.
"There were no policemen there because there was no reason. We are now conducting an investigation to see who was responsible," he said.
The protests have caused hundreds of millions of dollars of damage in Athens alone and have shaken the increasingly unpopular conservative government.
An opinion poll published on Thursday by Avgi, a left-wing newspaper, conducted after the start of the riots, showed the opposition Pan-Hellenic Socialist Party 6.5 percentage points ahead of the ruling New Democracy party.
Government officials say the violence has tarnished Greece's image overseas.
The policeman who shot 15-year-old Alexandros Grigoropoulos has been charged with murder, and jailed pending trial, while his partner was charged as an accomplice.
He says he fired a warning shot in self-defence against a group of youths but the family's lawyer said he aimed to kill without significant provocation.