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.

More violence feared

Barnaby Phillips, Al Jazeera's correspondent in Athens, said the situation was calm but that there was a possibility of further trouble later on Monday.

"The Communist Party has called for a rally in the centre of the city and it is possible that other far left groups may join them," he said.

Overnight, rioters hurled molotov cocktails and rocks, smashed shop windows, torched cars and erected burning barricades in the capital and Greece's second city of Thessaloniki.

Protesters chanted "cops, pigs, murderers" and rained petrol bombs down on riot police, while helicopters hovered overhead and officers used tear gas.

Small groups of youths continued to occupy university campuses in central Athens on Monday, throwing stones and molotov cocktails at police.

Violence also erupted in Hania, Crete and across other cities in northern Greece.

Police said that 24 officers were injured in Athens since the rioting began on Saturday night, with another 13 hurt in clashes on Sunday.

Seven people have been arrested and another 15 detained, according to news agency Associated Press.

Officers charged

The wave of violent protests was triggered by the police shooting of Alexandros Grigoropoulos in the traditionally left-wing Exarchia district on Athens on Saturday.

"It seems that the police are succeeding in holding the rioters back... the air is thick with tear gas"

Barnaby Phillips, Al Jazeera's correspondent in Athens

Two officers have been charged over the shooting - one with premeditated manslaughter and the illegal use of a weapon and the other as an accomplice. 

The are due to appear before a court on Wednesday and both have been suspended - along with the Exarchia precinct police chief.

A police statement said that one of the officers had fired three shots after their car was attacked by around 30 youths.

Prokopis Pavlopoulos, the Greek interior minister, has promised a full investigation into the shooting.

"It is inconceivable for there not to be punishment when a person loses their life, particularly when it is a child," he said.

"The taking of life is something that is not excusable in a democracy."

Resignation offer

However, earlier on Sunday the interior minister denounced the violence as "against human rights" and defended the police response, saying "no rage, even justified, must lead to protests like those we saw [on Saturday]".

Two officers have been charged over the shooting of Grigoropoulos
Pavlopoulos's offer to resign was rejected on Sunday by Costas Karamanlis, the Greek prime minister.

Karamanlis's embattled government has lost three ministers amid corruption scandals in the last 12 months.

Our correspondent, Barnaby Phillips, said the government and the police appeared to have been caught "on the backfoot" by the violence.

"The centre-right government was already in trouble and is clinging on to power with a very small majority.

"Ministers have been on TV apologising for the death of the boy as much as condemning the widespread looting and violence which is indicative of how the government is on the backfoot," he said.

Phillips also said recent political history made it difficult for the police to tackle the rioters.

"Since the overthrow of the military junta in the 1970s, Greek public opinion simply just doesn't tolerate a strong, authoritarian display of force. To some extent, the Greek police always have their hands tied," he said.