Pitched battles involved tear gas and petrol bombs
Protests are expected for a third day in Greece, following rioting over the fatal shooting of a teenager by police.
In cities across the country, dozens of protesters and police were injured during pitched battles on the streets, involving petrol bombs and tear gas.
Some protesters stayed in universities on Sunday night, while leftist groups planned rallies on Monday.
The government has called for an end to the violence, saying justice had been served with the arrests of policemen.
The riots began on Saturday after 15-year-old Andreas Grigoropoulos was shot dead by police in the Exarchia area of Athens.
Among protests called on Monday was a rally by the Greek Communist Party and the socialist PASOK opposition, in Athens.
"We must answer the government's policies en masse and peacefully," the PASOK youth branch said in a statement quoted by Reuters news agency.
MAJOR RIOTING IN GREECE
1973 - Brutal repression of student uprising in Athens helps bring down the military junta
1985 - Youths clash with police in Athens after rally marking 1973 uprising becomes violent and police shoot dead 15-year-old boy
1991 - Riots break out across the country after a school teacher is killed during protests in Patras
1995 - Riots erupt after protest in Athens and revolt in prison ahead of 1973 uprising's anniversary
1999 - Police clash with protesters opposing a visit by US President Bill Clinton to Athens
2003 - Youths battle police during an EU summit in Thessaloniki
2008 - Protesters battle police across country after an officer shoots dead a teenager in Athens
University professors said they would start a three-day walkout on Monday, rather than joining a nationwide workers' strike against pension reforms and economic policies on Wednesday, the agency said.
As night fell on Sunday, groups of protesters used rubbish bins and overturned cars to erect burning barricades in the streets around the Polytechnic, inside whose campus many have taken refuge in the knowledge that by law police are prohibited from entering.
Police said the first day of riots had left 24 police officers injured, one seriously, and 31 shops, nine banks and 25 cars damaged or burned.
Six people were arrested, one of them for carrying a weapon.
The unrest, the worst in the country in several years, later spread to Thessaloniki, Patras, and the islands of Crete and Corfu.
A march by more than 1,000 people on two police stations in Thessaloniki descended into violence when protesters attacked police and shops with firebombs and rocks.
Prime Minister Costas Karamanlis has written to Andreas Grigoropoulos's parents expressing his profound sorrow.
Interior Minister Prokopis Pavlopoulos, whose offer to resign was refused by the prime minister, has urged both protesters and police to act with restraint.
MAIN LOCATIONS OF ATHENS PROTESTS
Thousands of students, leftist demonstrators and anarchists on Sunday marched from the National Archaeological Museum and Polytechnic on Patission Avenue towards the police headquarters on Alexandras Avenue
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