Greece remains tense as it prepares for a third day of demonstrations sparked by the shooting dead of a 15-year-old boy by police.
Thousands of rioters fought running battles with riot police in cities across the country on Sunday, leaving at least 34 people injured.
The socialist Pasok opposition party called on Greeks to denounce the government, but refrain from any further violence, ahead of a rally mass rally in central Athens organised by the Greek communist party.
"We must answer the government's policies en masse and peacefully," the youth branch of Pasok said in a statement.
University professors, who had planned to join a nationwide strike against pension reforms and economic policies on Wednesday, said they would now stage a three-day walkout starting on Monday.
Internet blogs popular with high school students urged them to stay away from class.