French President Emmanuel Macron signed his controversial pension reform into law on Saturday, prompting accusations from unions and the left he was showing "contempt" towards those behind a three-month protest movement.

The alterations became law after the text was published before dawn in France's official journal, with the livid opposition claiming that Macron had moved to smuggle it through in the depth of the night.

The publication came just hours after the approval on Friday by the Constitutional Council of the essence of the legislation, including the headline change of raising the retirement age from 62 to 64.

The standoff with unions and left has become the biggest challenge of the second mandate of Macron, who will now address France on the crisis on Monday evening, the Elysee said.

Unions warned they were calling for mass Labour Day protests on May 1, and sometimes violent demonstrations erupted in several cities overnight after the verdict was announced.

The nine-member Constitutional Council ruled in favour of key provisions of the reform, including raising the retirement age to 64 and extending the years of work required for a full pension, saying the legislation was in accordance with French law.

Six minor proposals were rejected, including forcing large companies to publish how many over-55s they employ, and the creation of a special contract for older workers.

The appearance of the text in France's Official Journal – the gazette of record – means it has now been enacted into law. (AFP)