French taxi unions have once again staged a nationwide strike, shutting down roadways across Paris in a protest against ride-hailing companies like Uber. Thousands of taxi drivers are expected to participate in demonstrations across Paris today, disrupting traffic to and from the French capital's two major airports. Protesters burned tires at a major thoroughfare on the western edge of Paris, where police used tear gas to disperse some, and two taxi drivers were injured after a shuttle bus drove through a blockade at Orly airport, French media reported this morning. Paris police say 20 arrests have been made so far.

The unions are calling for an end to non-taxi services that, like Uber and other ride-hailing apps, allow users to book rides with licensed chauffeurs — a category known in France as "voitures de tourisme avec chauffeur" (VTC). A 2014 law aimed at assuring fair competition between VTCs and taxi operators has not been adequately enforced, the unions say, allowing VTCs to eat away at their market. CGT Taxis, one of the syndicates that organized today's strike, says its revenue has dropped by 40 percent due to the rise of ride-hailing apps. Others have estimated their losses at between 20 and 30 percent.

"we are demanding the elimination of VTCs, pure and simple."

"The law has never been enforced, and we think it's unenforceable," says Mohammed Khamedi, secretary of CGT Taxis. "So we are demanding the elimination of VTCs, pure and simple, or compensation" for taxi licenses. (Taxi licenses are issued for free but the government has limited the number in circulation, giving rise to a secondary market where they sell for around €200,000.)

Tuesday's demonstrations coincide with a series of strikes staged by air traffic controllers, civil servants, and teachers across France. It also comes more than a year after French taxi unions staged widespread protests against Uber's low-cost UberPop service (known as UberX in the US), which connected passengers with non-professional chauffeurs. Strikes held in January and June of last year erupted in violence, as taxi drivers set cars on fire and clashed with Uber chauffeurs. Uber suspended the service following the June protests, and it was officially banned under a court ruling handed down in September.

The French government created the VTC class in 2009, as a way to supplement the scarce supply of taxis. At first, the distinction was clear: VTCs could only be reserved in advance, whereas taxis could be hailed from the street. But the rise of apps like Uber, which now has 1.4 million users in France, has dramatically shifted that balance, putting taxis and private chauffeurs in more direct competition with one another.

Uber France spokesman Thomas Meister says today's strike "is not aimed at Uber" specifically, unlike previous demonstrations, saying the taxi unions are targeting "the general organization and structure of the industry." In an email to its French users on Monday, the San Francisco-based company described the strike as an assault on France's growing market for ride-hailing apps, and sought to muster online support to ease regulations.

"The objective of this protest is simple: pressure the government to make it more difficult to access the VTC profession so as to limit competition, while the sector is booming," the email read. The message was signed by Uber and five other French ride-hailing apps, and was sent to the users of all six startups. It was headlined with the hashtag "#NonALaFinDesApplis" ("No to the end of apps"), and urged users to sign a petition asking President Francois Hollande to ease testing restrictions for would-be VTC drivers. The government was supposed to implement new testing requirements for VTC chauffeurs by the beginning of 2016, replacing a 250-hour driving requirement, but it has yet to do so, leaving thousands of applicants in limbo. The site for applicants has been down for several days, displaying a message that says a new site will be available on January 26th (the date of today's strike).

Uber has faced regulatory hurdles throughout the course of its global expansion, and has encountered particularly hostile resistance in France. At the core of this week's strike is the Thévenoud law, passed in October 2014, which imposes restrictions on the way Uber and other ride-hailing apps operate. Under the law, VTC services are forbidden from using geolocation services to show available cars, and chauffeurs are required to return to a home base between rides.

The law was designed to ensure fair competition between France's deeply entrenched taxi firms and an emerging field of Uber-like VTC services. But neither side is happy with it. Uber believes the law is unfair and impractical — the company has challenged it in European court — while taxi unions say the government hasn't done enough to enforce it, and that it's regularly flouted.

"The Thévenoud law has given taxis the illusion that it would basically protect their monopoly, and that's simply not the fact," Meister said in an interview Monday. "It's just trying to create some balance, in a very clumsy way, in the industry. But taxis are absolutely sure that if the law were to be enforced, then they would have their monopoly back, which is obviously not going to happen."

Last week, Uber announced it would open its platform to taxi drivers, after France's Constitutional Court ruled that chauffeurs can work for both cab companies and VTC services, overturning a provision in the Thévenoud Law. Meister says the expansion would allow taxi drivers to have a second source of income, but it isn't sitting well with taxi unions. Khamedi describes the move as a "provocation," and says his syndicate will not work with Uber.

"We don't work with any company that doesn't participate in the social system in France."

"We don't work with any company that doesn't participate in the social system in France," he says. "They don't have the same constraints."

The government has so far given no indication as to how it will respond to the taxi unions' demands. Following June's protests, two Uber executives were arrested on charges of operating an illegal taxi operation. Ahead of this week's protests, the founders of Heetch, a late-night ride-sharing service for young people, were also taken into custody on similar charges.

"The violence is unacceptable," French Prime Minister Manuel Valls told reporters Tuesday. "No cause can justify such violence."

Although unions have historically wielded considerable influence over policymaking, France's economy minister has made a point of nurturing the country's startup industry, as it continues to struggle with high unemployment and anemic growth. Valls, Interior Minister Bernard Cazeneuve, and other officials will meet with a delegation of taxi drivers Tuesday afternoon.

"For me, [the taxi strike] is only noise," says Théodore Monzies, founder of the startup Eurecab, a price comparison and ride-booking site for car services in France. He says apps like Uber have blurred the distinction between VTCs and taxi services, and there's little the government can do to restore it. "The trend is moving forward, and I don't see any backward movement."

But the taxi unions, as always, remain determined. "If the government doesn't meet our demands, we will continue," Khamedi says. "We will stay on-site."

6:43AM ET: This article has been updated to include comments made by French Prime Minister Manuel Valls.