French protesters launched a new push on Tuesday to pressure President Emmanuel Macron into dropping a pension reform plan, with hundreds of thousands taking to the streets and strikes disrupting transport and schools.

Union-led protesters came out for mass demonstrations for the second time in less than two weeks, challenging Macron's plan to raise the age of retirement from 62 to 64, a flagship reform of his second mandate.

Half a million people were protesting on Tuesday afternoon in Paris alone, the main CGT union said, higher than the figure of 400,000 it gave for the last day of rallies on January 19.

A police source said the authorities were bracing for up to 1.2 million people to take to the streets nationwide, which would exceed the 1.1 million who came out on January 19.

But Macron has shown no sign of stepping back, insisting on Monday that the reform was "essential".

Transport worker Arnaud Roure, 47, was among those marching.

"Ladies and gentlemen from the government, you are wearing people down, you are sucking up all the resources we had left," he said. "You are attacking our bread."

The first marches had started in the morning in other parts of the country, with several prominent opposition politicians taking part.

"Mr Macron is certain to lose," said Jean-Luc Melenchon, a figurehead for the far left and former presidential candidate, as he marched in the southern port city of Marseille.

The most controversial part of the overhaul is hiking the minimum retirement age, but it also calls for more years worked to qualify for a full pension.

"I don't want to work longer," said Sylvie Dieppois, 56, a kitchen helper near Rouen in western France. "My job is hard and even at 62 I will be exhausted."

France has the lowest qualifying age for a state pension among major European economies.

Millions had to find alternative means of transport Tuesday, work from home or take time off to look after their school-age children, with workers in transport and education sectors among those staging walkouts.

"This is about more than pensions, it is about what kind of society we want," 59-year-old university professor Martine Beugnet told AFP.

Paris metro and suburban rail services were severely restricted, as was intercity travel.

A union source told AFP that 36.5 percent of staff at railway operator SNCF had stopped work, a figure down from 46.3 percent on January 19.

Around a quarter of all nursery and primary school teachers were on strike, according to the education ministry. In middle and high schools, more than half of teachers had stopped work, a teachers' union said.

France's oil industry was mostly paralysed, with the CGT union at energy giant TotalEnergies reporting between 75 and 100 percent of workers on strike.

Almost two out of 10 civil servants were striking by midday, the authorities said, down from 28 percent on January 19.

High school and university students also joined the movement, with a few dozen students at the prestigious Sciences-Po university occupying its main building overnight.

"It's important to get young people involved in the pensions debate," student Jean-Baptiste Bonnet said.

Even a prison, in the southwestern city of Nimes, was blocked by protesting staff, a union source said.

Sixty-one percent of French people support the protest movement, a poll by the OpinionWay survey group showed on Monday -- a rise of three percentage points from January 12.

"The more French people find out about the reform, the less they support it," said Frederic Dabi, a prominent pollster at the Ifop institute.

"This is not good at all for the government."

The government has said the changes are necessary to guarantee the future financing of the pension system, which is forecast to tip into deficit in the next few years.

But opponents point out that the system is not in trouble, insisting pension spending is not out of control.

The government has signalled there could be wiggle room on some of the suggested measures, but not on the age limit.

Macron's centrist allies, short of an absolute majority in parliament, will need votes from conservatives to push through the new legislation. (AFP)