PARIS (Reuters) - Groups of young men with faces masked, some carrying metal bars and axes, rioted on the streets of central Paris on Saturday, setting a dozen vehicles ablaze and torching buildings, unleashing the city’s worst urban unrest for years.

Firemen extinguish burning cars set afire by protesters wearing yellow vests, a symbol of a French drivers' protest against higher diesel fuel taxes, during clashes near the Place de l'Etoile in Paris, France, December 1, 2018. REUTERS/Stephane Mahe

Near the Ritz hotel, not far from high-end boutiques and restaurants, and in the avenues off the Arc de Triomphe, where several foreign embassies are located, gangs of violent protesters ran riot, setting a police van on fire and overturning cars. At least two buildings were aflame.

Close to the Place Vendome, Christmas trees decorating the streets were upended, piled in the middle of an avenue and set alight, prompting chanting from scores of protesters.

Youths smashed the windows of the flagship Apple Store on the Champs Elysees which opened just a few weeks ago. A boutique on the Rue Rivoli was broken into and looted. Banks were vandalized and scrawled with anti-government graffiti.

Police said several hundred far-right and far-left extremists had infiltrated a demonstration by some 5,000 calmer “yellow vest” protesters who had gathered in the capital to denounce President Emmanuel Macron’s fuel tax increases.

“We are in a state of insurrection, I’ve never seen anything like it,” Jeanne d’Hauteserre, the mayor of the city’s 8th arrondissement, near the Arc de Triomphe, told BFM TV.

It is the third week of national unrest by the movement -- named after the high-visibility jackets that all motorists in France must carry in their vehicles -- and the second consecutive weekend of violent disturbance in Paris.

As night fell on the city, there were overturned vehicles in flames around the Arc de Triomphe, an assault rifle had been stolen by protesters from a police van, and fires were burning in popular tourists areas such as Opera.

Around Saint Lazare station, the windows of a bank were smashed before police on horseback moved down the main street, pushing protesters back. A branch of an insurance company was sprayed with graffiti reading “Macron in prison”.

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Near Madeleine, another popular tourist area, a BNP bank was vandalized and an ATM machine pulled out of the wall. Graffiti was scrawled on the building reading “Macron the clown” and “Who sows misery reaps anger”. The nearby Dior and Chanel stores had their windows shattered.

In some areas there was very little police presence. Reuters reporters on the streets witnessed masked young men, one brandishing a crowbar, smashing up a police van and then setting it alight. Others stood on top of the vehicle in celebration.

Firemen arrived to douse the flames. But only minutes later, another group of young men arrived, and after failing to overturn the van they detonated firecrackers inside it, causing a series of explosions.

Tourists near the famed Galeries Lafayette department store were alarmed by the uncontrolled outpouring of violence.

“We went to the Galeries Lafayette as we thought we would be safe there but then we were evacuated by staff and security,” said Tina Holten, a 35-year-old visitor from Denmark. “We felt safe inside but now that we are outside we are very scared. We can’t find anywhere to go.”

On the Rue de la Paix, one of Paris’s most expensive shopping streets, Christmas trees were left smouldering after firemen put out at least three blazes. The jewelry stores and fashion boutiques were locked up, but the Christmas decorations on the street were still sparkling.

“The problem is that you just don’t know where it’s going to flare up next,” said Caroline Quevrain, a 23-year-old student. “It’s chaotic but the protesters wanted it that way.”

The U.S. embassy issued a statement urging citizens to be careful, saying that “violent clashes between police and protesters” continued in at least three of Paris’s 20 districts, known as arrondissements.

“Avoid all demonstrations, seek shelter in the vicinity of clashes, follow instructions of security personnel,” it said.

It was unclear what plans the police had to shut down the roving groups of protesters. Near the Arc de Triomphe, they moved in to try to control the violence, but in other areas of the city, there was next to no presence of security forces, although there were signs of the violence dying down.

It is the worst unrest Paris has seen since at least 2005, when the outskirts were in flames over tension between the police and the youth of the poor suburbs known as “banlieues”, demonstrating over the death of two young people.