French police in riot gear used tear gas as taxi drivers stepped up protests against US online lift-sharing service UberPOP, blocking road access to airports and train stations in Paris and other cities.

Aeroports de Paris, the operator of the French capital's Charles de Gaulle and Orly airports, warned travellers to take local train services because of the taxi protest.

"Access by road is completely blocked," the company said on its website. "The only way to get to CDG is (by train)."

French media showed images of burning tyres blocking part of the ring road around central Paris, as well as scuffles between protesting taxi drivers and other motorists, while police in riot gear at one point intervened using tear gas.

Taxi drivers also blocked access to Paris' Gare du Nord train station, from where the high-speed Eurostar and Thalys services run to London and Brussels.

Drivers set up barriers around Marseilles and Aix-en-Provence in southeast France, including at key motorway exits, and blocked access to train stations in the two cities.
They were also protesting on the main access to Marseilles-Provence airport.

Uber, which links drivers with passengers through a smartphone app, has been expanding its UberPOP service in French cities, provoking anger from taxi drivers who see it as unfair competition.

The service links drivers of private cars with potential passengers at cheaper rates than traditional cabs.

UberPOP is also a more informal operation than private chauffeured tourism vehicle services called VTCs that use professional drivers.

"We are faced with permanent provocation (from Uber) to which there can only be one response: total firmness in the systematic seizure of offending vehicles," G7 taxi firm head Serge Metz told BFM TV.

"We are truly sorry to have to hold clients and drivers hostage. We're not doing this lightly."

UberPOP has been present in Paris since 2011 and has expanded to other cities, but faces a legal battle.

A law from October 2014 placed a ban on putting clients in touch with unregistered drivers. Uber has contested the rule, saying it is unclear and counter to the freedom to do business.

On Tuesday the matter was referred to France's Constitutional Council, a court with the power to review the consitutionality of legislation.