A strike by air traffic controllers in support of the protest saw 30 per cent of flights from Paris Orly airport cancelled and 10 per cent from Charles de Gaulle, the environment ministry, said.
"What happens today will be fairly decisive for how things develop," said Bernard Thibault, leader of the CGT, the largest of the broad coalition of trade unions organising the national protest.
"I'd like to see us exceed the mobilisation we achieved on March 23," he told Europe 1 radio, referring to France's last large-scale labour protest, when unions estimated turnout at 800,000 and the police at 350,000.
If the unions fail to mobilise a similar number this week, it is likely to be seen as a victory for the government, but labour and opposition leaders said they were confident of a big turnout.
The postal service said that 12.58 per cent of staff were on strike, slightly more than the 11.45 per cent who walked out on March 23.
Polls published on Thursday in two newspapers, Le Parisien and L'Humanite, found that around two thirds of French voters were prepared to join one of the dozens of rallies being organised around the country.
This appeared to reflect growing opposition to Sarkozy's plan, which the government only confirmed this week.
A previous poll conducted this month by CSA/CECOP showed a narrow majority accept the change is inevitable, whereas a later survey found a similarly narrow majority think it unnecessary.
In common with much of Europe, France is grappling with a huge public deficit, and the government argues that reforming pension rules and delaying the minimum retirement age will help control mounting debt.
Many of France's neighbours have announced harsh spending cuts but Sarkozy, who is suffering record unpopularity and faces a re-election fight in two years, has been cautious, refusing to speak of an austerity programme.
Nevertheless, ministers confirmed this week that they planned to abolish retirement at 60, a cherished symbol for the French left of its victories under Francois Mitterrand, the late president.
French retirees receive 85 per cent of their pension payments from state schemes, compared to an average of 61 per cent among member states of the Organisation for Economic Co-operation and Development (OECD).
Although 60 is the theoretical minimum age for retirement on a full state pension, various special schemes exist in the public sector for those with jobs perceived as tough or those who started in work in their teens.
On average, French men retire at 58.7 years and women at 59.5, compared to an OECD average of 63.5 and 62.3, the OECD reports.
"It's a demographic problem. France is behind Malta as the country where we work the least," said Francois Baroin, the budget minister.
Pensions account for the bulk of the social security budget, which can no longer in itself cover payments, with the excess being covered by state borrowing, forcing up France's public deficit.
According to the French government's panel studying pension finance, the shortfall between pension contributions and spending was $13.4bn in 2008 and will rise to between $87.8bn and $140.3bn by 2050.