* Yellow vests converge in Paris to protest high living costs
* Militants hurl projectiles at riot police near Champs-Elysees
* Police fire tear gas, stun grenades, water cannon
* Protesters skirmish with police in other French cities
* Officials feared infiltration by far-left, far-right groups
* Macron says understands anger but sticking to reforms (Adds situation calm, prime minister trip cancelled)
PARIS - Rioters ran amok across central Paris on Saturday, torching cars and buildings, looting shops, smashing windows and clashing with police in the worst unrest in more than a decade, posing a dire challenge to Emmanuel Macron's presidency.
The authorities were caught off guard by the escalation in violence after two weeks of nationwide protests against fuel taxes and living costs, known as the "yellow vest" movement after fluorescent jackets kept in all vehicles in France.
In Paris, police said they had arrested almost 300 people while 110 were injured, including 20 members of the security forces. Police fired stun grenades, tear gas and water cannon at protesters at the top of the Champs-Elysees boulevard, at the Tuilleries Garden near the Louvre museum and other sites.
In some areas there was virtually no police presence at all, as groups of masked men roamed in the shadows of the capital's fabled landmarks and through its fanciest shopping districts, smashing the windows of designer boutiques.
Macron, in Argentina for a G20 summit, said he would convene ministers to discuss the crisis upon his return on Sunday. Prime Minister Edouard Philippe cancelled a trip to Poland.
"We are in a state of insurrection, I've never seen anything like it," said Jeanne d'Hauteserre, the mayor of Paris' 8th district, near the Arc de Triomphe.
The popular rebellion erupted out of nowhere on Nov. 17 and has spread quickly via social media, with protesters blocking roads across France and impeding access to shopping malls, factories and some fuel depots.
On Saturday, some targeted the Arc de Triomphe, chanting "Macron Resign" and scrawling on the facade of the towering 19th-century arch: "The yellow vests will triumph."
Addressing a news conference in Buenos Aires, Macron said no cause justified the looting of stores, attacks on the security forces or torching of property. The violence, he said, had nothing to do with the peaceful expression of legitimate grievances.
"I will always respect differences. I will always listen to opposition, but I will never accept violence," Macron said.
Protesters smashed the windows of a newly opened flagship Apple Store and luxury boutiques of Chanel and Dior, where they daubed the slogan "Merry Mayhem" on a wooden board.
Close to the Place Vendome, Christmas trees decorating the streets were upended, piled in the middle of an avenue and set ablaze, prompting chanting from scores of protesters.
Order appeared to have been restored late in the evening, although small groups were still at odds with police near the Champs Elysees.
Authorities said violent far-right and far-left groups had infiltrated the yellow vests movement. Interior Minister Christophe Castaner said most of those arrested were regular protesters who had been egged on by the fringe groups.
The protests began as a backlash against Macron's fuel tax hikes, but have tapped into a vein of deep dissatisfaction felt towards the 40-year-old's economic reforms, which many voters feel favour the wealthy and big business.
Unrest erupted in several towns and cities across France, from Charleville Mezieres in the northeast to Marseille in the south. In the Riviera city of Nice trucks blocked access to the airport, and in the central town of Puy-en-Velay the police headquarters was set on fire.
The protests are taking a toll on the economy. Parts of central Paris that should have been packed with tourists and Christmas shoppers resembled battle zones, as smoke and tear gas hung in the air and debris littered the ground. Hotels and department stores in the capital stand to lose millions, and shelves have run empty in some supermarkets.
MACRON STANDS FIRM
The protests have caught Macron off-guard just as he was trying to counter a fall in his popularity rating to 20 percent. His unyielding response has exposed him to charges of being out of touch with ordinary people, particularly in rural villages and the provincial hinterlands.
Some peaceful protesters held up a slogan reading, "Macron, stop treating us like idiots!"
Macron on Tuesday said he understood the anger of voters outside France's big cities over the squeeze fuel prices have put on households. But he insisted he would not be bounced into changing policy by "thugs".
Despite the unrest that has accompanied the protests, the "yellow vests" have widespread public support, even in cities.
"I am totally behind the 'Gilets Jaunes'," said George DuPont, a resident in Paris' upscale 16th arrondissement. "The state has stolen money from the French people. It's time to give it back."
Assistant teacher Sandrine Lemoussu, 45, who travelled from Burgundy to protest peacefully, said people were fed up with Macron.
"The people are in revolt," she said. "The anger is rising more and more, and the president despises the French. We aren't here to smash things, but the people have had enough."
Many on the outskirts of smaller provincial towns and villages have expressed anger, underlining the gap between metropolitan elites and working class voters that has boosted anti-establishment politics across the Western world.
"Mr Macron wrote a book called Revolution. He was prophetic because it is what he has managed to launch, but not the revolution he sought," Far-left La France Insoumise leader Jean-Luc Melenchon told reporters ahead of a protest in Marseille.
(Reporting by Thierry Chiarello, Antony Paone, Sudip Kar-Gupta, Bate Felix, Luke Baker, Sybille de la Hamaide, John Irish, Celia Mebroukine, Antoine Boddaert, Lucien Libert, Stephane Mahe, Caroline Paillez in Paris, Jean-Francois Rosnoblet in Marseille and Johanna Decorse in Toulouse; Writing by John Irish and Richard Lough; Editing by Mark Heinrich, Hugh Lawson and Peter Graff)