President Nicolas Sarkozy could face the greatest challenge of his presidency in the next few days as a month-old protest against pension reform swells towards outright confrontation.

More than 3 million demonstrators - one in 20 of all French people - marched yesterday against the President's plans to raise the standard retirement age from 60 to 62. Tens of thousands of students joined the marches for the first time, threatening to radicalise the protests and broaden them into a rebellion against a deeply unpopular presidency.

Militant union branches in the railway and oil-refining industries were pushing last night for a showdown with Mr Sarkozy, who has made reform of the loss-making state pension system the make-or-break issue of his final 20 months in office.

"The demonstration," headlines Libération after the fourth day of protests against the pension reforms in France. Somewhere between 1,230,000 (according to the police) and 3.5 million people (according to unions) took to the streets, including a great many high school and university students. The strike is to continue in certain sectors, including the port of Marseille that supplies six big oil refineries in the south of France. "The strikes are now renewable," reports Libération. "Under the umbrella of a majority-opinion movement, the most determined union activists, although a minority, are trying to block public transport and petrol deliveries. Invented in 1995 at the time of the Juppé reform [also of pensions], the `strike by proxy' is back." More protests are planned, moreover, but the government has let it be known that it will not backtrack on its decision to raise the retirement age to 62.