Unrest that has gripped Greece for the past six days showed troubling signs of spreading across Europe, as violence erupted in several cities.

Angry youths smashed shop windows, attacked banks and hurled bottles at police in small but violent protests Thursday in Spain and Denmark, while cars were set alight outside a consulate in France. Protesters gathered in front of the Greek Embassy in Rome on Wednesday and some turned violent, damaging police vehicles, overturning a car and setting a trash can on fire.

Authorities say the incidents have been isolated so far, but acknowledge concern that the Greek riots — which started over the police killing of a 15-year-old on Saturday — could be a trigger for anti-globalization groups and others outraged by economic turmoil and a lack of job opportunities.

"What's happening in Greece tends to prove that the extreme left exists, contrary to doubts of some over these past few weeks," French Interior Ministry spokesman Gerard Gachet told The Associated Press. "For the moment, we can't go farther with our conclusions and say that there's a danger of contagion of the Greek situation into France. All of that is being watched."

As Europe plunges into recession, unemployment is rising, particularly among the young. Even before the crisis, European youths complained about difficulty finding well paid jobs — even with a college degree — and many said they felt left out as the continent grew in prosperity.

At least some of the protests appear to have been organized over the Internet, showing how quickly a message can be spread, particularly among tech-savvy youth. One Web site that Greek protesters have been using to update each other claims there have been sympathy protests in nearly 20 countries.

"We're encouraging nonviolent action here and abroad," said Konstantinos Sakkas, a 23-year-old protester at the Athens Polytechnic, where many of the demonstrators are based. "What these are abroad are spontaneous expressions of solidarity with what's going on here."

In Denmark, protesters pelted riot police with bottles and paint in downtown Copenhagen at a rally late Wednesday. Some 63 people were detained and later released.

And in Spain, angry youths attacked banks, shops and a police station in separate demonstrations in Madrid and Barcelona late Wednesday that each drew about 200 people.

Some of the protesters chanted "police killers" and other slogans. Eleven people — including a Greek girl — were arrested at the two rallies, and two police officers were lightly injured.

The Barcelona daily La Vanguardia said the protests had been convened over the Internet.

Daniel Lostao, president of the state-financed Youth Council, an umbrella organization of Spanish youth groups, said young people in Spain face daunting challenges — soaring unemployment, low salaries and difficulty in leaving the family nest because of expensive housing.

Still, he said he doubted the protests in Spain would grow.

"We do not have the feeling that this is going to spread," Lostao said. "Let's hope I am not wrong."

In France, protesters set fire to two cars and a garbage can apparently stuffed with flammable material outside the Greek consulate in Bordeaux early Thursday and scrawled graffiti on the building threatening more unrest, Michel Corfias, the Greek consul, told The Associated Press.

"It was a very, very intense fire," Corfias said, adding that it severely damaged the building's front door.

Graffiti sprayed on the consulate's garage door read "solidarity with the fires in Greece, the insurrection to come," he said, and the word "insurrection" was painted on the doors of neighboring houses.

Corfias said police suspect the attack is linked to events in Greece, and that it might have been carried out by youths unhappy with globalization and economic difficulties in France.

"The events in Greece are a pretext, in my opinion," he said. "The events in Greece are a trigger."

Elsewhere in Europe, more than 15 people occupied a Greek consulate in Berlin on Monday, hanging a banner out the window with the dead Greek teenager's name and the words, "Killed by the State." Youths clad in black appeared occasionally at a consulate balcony, exchanging chants with more than 50 protesters gathered on the street below.

About 100 people protested outside the Greek consulate in Frankfurt on Tuesday evening and minor violence was reported on the peripheries of the demonstration, including the breaking of a bank's window.