France's public transport networks ground to a near halt as unions dug in for a nationwide strike aimed at forcing President Emmanuel Macron to abandon plans to overhaul the country's pension system.

Key points: The civil aviation authority asked airlines to cancel around 20 per cent of flights

The civil aviation authority asked airlines to cancel around 20 per cent of flights Many international and local train services have been cancelled

Many international and local train services have been cancelled Unions have set no end date for the strike

Several thousand people protested in Paris as unions, covering public sector workers including train operators and teachers, launched open-ended strikes on Thursday.

Small groups of masked activists, including Black Bloc protesters, smashed store windows, set fires and hurled flares on the sidelines of the otherwise peaceful protest, prompting volleys of tear gas from police in body armour.

Paris authorities barricaded the presidential palace and deployed 6,000 police as activists, many in yellow vests representing France's year-old movement for economic justice, gathered in the capital in a mass outpouring of anger at Mr Macron and his centrepiece reform.

The Eiffel Tower was shut down, the Louvre Museum warned of strike disruptions, and metro stations across Paris shut their gates.

Commuters dusted off old bicycles, turned to carpooling rides or worked from home.

Before sunrise, riot police deployed along the boutique-lined Champs-Elysees boulevard, searching the bags of pedestrians ahead of a day of street protests which the government has warned may be infiltrated by violent groups.

"Public transport will be very difficult today, as it will tomorrow and probably this weekend too," junior transport minister Jean-Baptiste Djebbari told RTL radio.

Small groups of protesters smashed store windows, setting fires and hurling flares as police fired tear gas. ( AP: Thibault Camus )

The countrywide walkout by public workers is one of the biggest strikes in decades.

Unions plan to cripple transport networks, close schools and leave garbage piled high to force Mr Macron into retreat.

The SNCF state railway says only one in 10 commuter and high-speed TGV trains will run. Train operators Eurostar and Thalys have cancelled at least half their services linking Paris with London and Brussels.

The civil aviation authority asked airlines to cancel around 20 per cent of flights because of knock-on effects from the strike.

People march during Thursday's demonstration in Paris. ( AP Photo: Thibault Camus )

Police carried out security checks of more than 6,000 people arriving for the protest and detained 65 even before it started.

Embassies warned tourists to avoid the protest area and many visitors cancelled plans to travel to one of the world's most-visited countries amid the strike.

Vladimir Madeira, a Chilean tourist vacationing in Paris, said the strike has been "a nightmare".

He hadn't heard about the protest until he arrived, and transport disruptions foiled his plans to travel directly to Zurich.

More than half of all primary and secondary teachers are expected to walk out and hospital emergency rooms nationwide will operate on thin staffing.

"What we've got to do is shut the economy down," Christian Grolier, a senior official from the hard-left Force Ouvriere union, said.

"People are spoiling for a fight."

Protesters will march from the capital's Gare du Nord train station to Place de la Nation in the afternoon (local time).

Interior Minister Christophe Castaner said thousands of anarchist "black bloc" and hardcore "yellow vest" protesters were expected to wreak havoc. He ordered shops along the route to close.

Pension reform a pain point

Mr Macron wants to simplify France's pension system, which comprises more than 40 different plans, many with different retirement ages and benefits.

He says the system is unfair and too costly, and wants to replace it with a unified scheme while using financial incentives to encourage people to work longer.

Unions say most workers would lose money with the new system due to a change in the calculation of pensions. People who qualify for special provisions fear the loss of benefits.

Past attempts at pension reform have ended badly.

Government officials have warned that hardcore "yellow vest" protesters such as these, pictured in March, are expected to wreak havoc during the December protests. ( AP: Christophe Ena )

Former president Jacques Chirac's conservative government in 1995 caved in to union demands after weeks of crippling protests.

Twenty-four years later, the looming standoff is a risky venture for France's unions that have seen membership and public support wane in recent years.

They are battling to remain relevant against a President who has faced down waves of strikes over reforms of the labour market and SNCF railways.

The Saint-Lazare train station in Paris remains empty as strikes begin. Tourists are cancelling travel plans and Paris is deploying thousands of police. ( AP Photo: Michel Euler )

For Mr Macron, the showdown will set the tone for the second half of his mandate, with more difficult reforms to come, including to unemployment benefits.

The strikes follow months of sometimes violent "yellow vest" protests over the high cost of living and the perceived elitism of the political class.

Laurent Berger, head of the reform-minded CFDT union, said the social environment was more explosive than in 1995.

"In terms of tension, social cohesion and fractures within society, it's a lot worse now," Mr Berger said.

Reuters/AP