France's opposition has attacked President Nicolas Sarkozy for signing into law a reform raising the minimum retirement age from 60 to 62.
The nationwide strikes and street protests which accompanied the reform's passage through parliament made it one of the biggest battles of his career.
Mr Sarkozy argued the reform was necessary because of rising numbers of pensioners and greater life expectancy.
But Socialist leader Martine Aubry accused the president of "brutality".
"He thinks that by ploughing ahead regardless he is showing courage," she said.
"[True] courage would have been setting in place a reform which finally settles the pension problem."
The changes also mean those who want to claim full pension benefits must now wait until the age of 67 instead of 65.
The act became law when it was published on Wednesday, a day after being approved by France's constitutional court.
The measures will come into effect gradually from July of next year, and be fully enacted by 2018.
French workers will now have to work 41.5 years to receive a full state pension.
Unlike previous French presidents, Mr Sarkozy stood his ground, refusing to bend in the face of fierce opposition from the streets, the BBC's Christian Fraser reports from Paris.
His signature on the bill was always a formality, particularly since the parliamentary process had been fast-tracked to successfully draw the heat from the protest, our correspondent says.
There is no doubt the passage of the bill is a defining moment in Mr Sarkozy's presidency - it was a key part of his reform agenda and his credibility was on the line.
And his haste in signing the bill into law was surely connected to his burning impatience to reshuffle his cabinet in a bid to improve his own record low approval ratings, our correspondent adds.
'He does not listen'
Trade unions held an eighth round of rallies and marches in French cities on Saturday but numbers were sharply down on previous demonstrations.
Both government and union estimates showed that Saturday's attendance was about a third that seen in mid-October.
Nonetheless, the unions have called another day of action for 23 November.
At their height, the French protests were the biggest seen in the EU during the current wave of austerity measures to cut debt and budget deficits after the global economic crisis.
Over two months of strikes and demonstrations caused chaos around the country, including blockades at fuel depots and oil refineries.
Cynthia Lerenard, a 22-year-old student demonstrating in Lyon on Saturday, said the protest was as much about President Sarkozy as his legislation.
"I'm here against the pension reform but I am also against the dictatorial methods of all the government reforms," she said.
"He does not listen to people," the protester added.