Groups of youths remain holed up in a university building where police officers are barred entry under a tradition of university asylum.
Nicole Itano, Al Jazera's correspondent in Athens, said: "People are expecting that once again this gathering of people will erupt into clashes between protesters and police.
"Because over the past few days, any time large numbers of people have gathered to protest, or even at the boy's funeral yesterday, they have generally started out peacefully but have quickly degenerated into violence, into clashes with police."
She said that the government had appealed to the union to put off the strike - which had been planned weeks in advance - in view of the riots, but union leaders said they would only scale back the protest.
"There is a very strong police presence particularly in the city centre right now."
The killing of 15-year-old Alexandros Grigoropoulos on Saturday triggered the most violent riots in the country in a quarter of a century and dealt a major blow to the country's increasingly unpopular conservative government.
Protesters clashed with police guarding the Greek parliament a day earlier as thousands of people attended the youth's funeral in the capital.
Hundreds of demonstrators threw petrol bombs and other projectiles at the police on Tuesday, before riot police forced them back towards Syntagma square in Athens.
1974 - Student uprising leads to the end of seven years of brutal military dictatorship. Police banned from entering university campuses.
1981 - A socialist government comes to power, rules for 23 years.
2004 - Conservative leader Costas Karamanlis elected. His administration becomes increasingly unpopular, for its handling of the 2007 deadly summer fires, a land-swap scandal and growing dissatisfaction with social and economic reforms.
The central square had been the scene of violence on Monday night when rioters torched shops, government ministries and banks.
The killing of Grigoropoulos has touched a raw nerve among young Greeks angry at years of political scandals and rising levels of poverty and unemployment, worsened by the global economic downturn.
Stathis, a police spokesman, said 89 people had been arrested for attacks on police, vandalism and looting since the violence began.
Another 79 people had been detained for questioning over the riots in which dozens of police officers have been injured.
At Grigoropoulos's funeral at the municipal cemetery of Palio Faliro, a residential suburb in southern Athens, some groups shouted anti-police slogans but it was mainly calm after a request from the family that respect be shown for the dead teenager.
Karolos Papoulias, Greece's president, had appealed for calm, calling on Greeks to "honour Alexis' memory peacefully".
"This is a day of mourning for us all ... but there must be respect for institutions and laws," he said in a statement.
The lawyer for two officers accused of shooting dead Grigoropoulos said on Wednesday that ballistics show he had been killed by a ricochet and not a direct shot.
|Protesters threw fiery projectiles at the
police outside parliament [AFP]
One of the officers has been charged with premeditated manslaughter and the illegal use of a weapon and another as an accomplice.
They are due to appear before a court on Wednesday and both have been suspended - along with the Exarchia precinct police chief.
Demonstrations and violence played out in similar ways in other cities across the country as George Papandreou, the Socialist opposition leader, demanded that the conservative government step down and call elections to help end the violence.
Addressing his parliamentary group on Tuesday, Papandreou said: "We claim power. The only thing this government can offer is to resign and turn to the people for its verdict."
Costas Karamanlis, Greece's prime minister, has vowed to end the country's worst unrest in decades, but a government spokesman denied reports that the government planned to declare martial law.