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.
Socialist leader Olivier Faure said that Macron's swift signing of the law showed "disdain" towards the protest movement while hard-left MP Francois Ruffin called it a "democratic hold-up".
"A law enacted in the middle of the night, like thieves," tweeted French Communist Party chief Fabien Roussel. "Everyone on the street May 1."
"Macron tried to intimidate all of France in the middle of the night," tweeted hard-left leader Jean-Luc Melenchon, calling Macron a "thief of life" who showed an "absurd display of arrogance".
The night-time announcement "confirms the violent contempt of the president both for the population and in particular for the trade unions," said the leader of the CGT union Sophie Binet.
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