Authorities in France are braced for the biggest mass demonstration in years as protesters take to the streets Saturday to decry President Emmanuel Macron’s proposed pension reforms.

Up to 850,000 people are expected to show up for protests scheduled to take place across the country.

Jean-Luc Mélenchon, the former presidential candidate and veteran left-wing leader, predicted that the mobilization would be “the biggest social mobilization in this country in a half a century.”

Macron’s political legacy could rest on the outcome of his plan to raise France’s retirement age to 64, as he tackles the thorny issue of pensions — a sacred cow of French society that is seen as the backbone of the country’s social contract.

Saturday’s action is the fourth in a series of protests, but union bosses are confident that it will be the biggest to date, given that it taking place on a weekend.

Marie Buisson, general secretary of the Ferc-CGT union, which represents the education and cultural sector, said Saturday morning that she hoped that many people who were unable to participate in previous stoppages during the week now would do so.

Macron put pension reform at the heart of his reelection campaign last year but is battling public backlash against his proposal to raise the minimum pension age, which is currently 62.

His new legislation went to parliament on February 6 and has been vilified by the left-wing NUPES parliamentary coalition, trade unions and broad swaths of the public.

French presidents have tried — and failed — for years to reform France’s pension system.

France, the EU’s second-largest economy, is out of step with much of Europe when it comes to its minimum retirement age. In neighboring Spain the retirement age is 65; in Germany, it was recently changed to 67, with many countries opting to increase their minimum retirement ages in recent years as people live longer and governments grapple with massive pension liabilities.

But critics of Macron’s plan have said that it risks disproportionately hitting lower-income workers and argue that there are other ways of tackling the pension deficit, such as taxing the super-rich.

Though Macron could technically push through the proposals through a special constitutional provision, the government has said it wants to get a majority in parliament, with economy minister Bruno Le Maire describing it as a question of “political legitimacy.”

As the political horse-trading steps up, Macron’s government has already made some concessions — proposing that those who began working at 20 and 21 can retire at 63, for example. But it is unclear if this will be sufficient to sway enough members of the National Assembly.