ATHENS, Greece (CNN) -- Authorities vowed to re-impose order after demonstrators rose up across Greece Monday in a third day of rioting over Saturday's killing of a 15-year-old boy that has left dozens injured and scores of properties destroyed.

"Under no circumstances will the government accept what is occurring," said Greek Interior Minister Prokopis Pavlopoulos. "We will do what is necessary."

It was unclear what would be necessary to placate the demonstrators. "We've just lost count of how many demonstrations are taking place now," a police spokesman in Athens told CNN.

Police said 34 civilians and 16 police officers were injured Monday in rioting that spread into new municipalities, including Trikala, Larissam and Veria.

Riots broke out Saturday in Thessaloniki and Athens, where police killed the teen. Watch the latest report on the rioting »

Demonstrators had torched three government buildings and three offices of the ruling conservative political party in downtown Athens, a National Fire Brigade spokesman told Greek state television. Watch as iReporter witnesses the clashes

Thirty-five cars and 160 trash containers also had been set ablaze, he said. See images of anarchy on Greek streets »

Demonstrators Monday barricaded streets in Athens and Thessaloniki and hurled gasoline bombs as they battled with police. Clouds of tear gas hung over the capital city as riot police continued to battle the hundreds of young self-styled anarchists rioting over the boy's death.

"Rage is what I feel for what has happened, rage, and that this cop who did it must see what it is to kill a kid and to destroy a life," a student in Athens told reporters Monday. Watch protesters clash with police »

In a nationally televised address broadcast on state television, Greek Prime Minister Kostas Karamanlis condemned the violence and promised to punish those responsible for Saturday's shooting.

He also announced a decision to drop plans to reimburse business owners affected by the rioting.

The police officer who fired the fatal shot has been charged with "manslaughter with intent" and suspended from duty, police said, adding that a second police officer was arrested Saturday on criminal accessory charges.

Government officials have condemned the shooting.

"An investigation is under way and those found responsible will be punished," said Pavlopoulos. "Measures will also be taken to avoid such incidents again in the future." iReport.com: Are you there? Share photos, video of rioting

On Monday, authorities conducted an autopsy on the teenage boy in an effort to answer questions about the circumstances of the shooting, but the boy's family has called in their own investigators to verify state findings, the Athens coroner told CNN.

The U.S. and British embassies issued warnings to employees and tourists on Sunday, instructing them to avoid downtown Athens and other major cities until rioting subsides.

Tourists in central Athens hotels were advised by hotel staff not to leave their rooms as police fanned out across the city.

"There are lots of burning bins and debris in the street and a huge amount of tear gas in the air, which we got choked with on the way back to our hotel," according to Joel Brown, a CNN senior press officer visiting Athens on Sunday.

A police statement about the teenage boy's death said the incident started when six young protesters pelted a police patrol car with stones. The teen was shot as he tried to throw a petrol bomb at the officers, police said.

Other angry teens converged on the site almost immediately.

Fighting between youths and police erupted elsewhere, including Thessaloniki, the country's second-largest city. Hundreds of young people took to the streets of the sprawling port city, finally barricading themselves behind the gates of a state university.

Authorities have been barred from entering university grounds since tanks crushed a 1973 student uprising protesting the ruling military junta. It was not clear what authorities would do about the demonstrators still holed up at the university.

No deaths have been reported since Saturday.

Police said Monday that 20 protesters had been rounded up for questioning.

Journalist Anthee Carassava in Athens contributed to this report.

All About Greece • Athens • Thessaloniki