More than 180 street protests were being held nationwide against the reforms, which are intended to tackle stubbornly high unemployment by loosening the rules that govern how businesses hire and fire people.
Some 4,000 strikes have been called under the action led by France's biggest trade union, the CGT, with rail workers, students and civil servants urged to protest in cities from Paris to Marseille and Toulouse.
The turnout will serve as a yardstick for unions' ability to mobilise, as deep splits have emerged in the labour movement between those determined to fight the reforms and those prepared to compromise.
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“What is going to be a surprise is when he (Macron) ends up giving ground,” far-left leader Jean-Luc Melenchon told reporters as he joined a protest in the southern port of Marseille.
“This country doesn't want the liberal world… France isn't Britain,” he added.
At lunchtime, crowds of a few thousand were reported in the cities of Nice, Marseille, Saint Nazaire and Caen, while a larger rally began in Paris later in the afternoon.
The business-friendly Macron sparked a backlash last week by describing opponents of the shake-up as “slackers” and cynics, in comments blasted as “scandalous” by CGT chief Philippe Martinez.
Bruno Cautres of the Cevipof political research institute said Macron had “thrown oil on the fire” with his choice of words.
“With the 'slackers' comment, there are all the ingredients for this to heat up,” he said.
Protesters seized on the remark on Tuesday, with some in the northern city of Caen shouting “Macron you're screwed, the slackers are in the street.”
In Paris, traffic was operating at between 50 and 80 percent of normal on suburban lines of the SNCF rail network, while two lines of the suburban RER metro system were hit.
Air traffic controllers have also been urged to strike, and Irish low-cost carrier Ryanair said Monday that it had cancelled 110 flights scheduled for Tuesday.
The airline called on Macron's government and the European Commission “to take immediate action to prevent the skies over Europe being closed yet again” during the stoppage.
Fairground operators — angry over pre-Macron reforms that open the industry to more competition — caused traffic jams Tuesday on a major motorway into Paris, and others used trucks to block roads in the capital itself, including on the Champs-Elysees Avenue.
More violence on the streets?
The 39-year-old centrist president, who swept to power in May on promises to reinvigorate the economy and transcend left-right politics, used executive orders to fast-track his labour reforms.
They must be ratified by parliament in the coming months, but are expected to breeze through given the large majority won in June by Macron's Republic on the Move party.
“This is not a labour law, it is a law that gives full powers to employers,” said the CGT's Martinez.
Eric Beynel of the Solidaires union, which backs the protests, vowed that workers would keep up the pressure “until the orders are withdrawn”.
But other unions have signalled a willingness to compromise, including the Force Ouvriere (FO) union, though some of its branches are planning to defy orders and down tools on Tuesday.
Macron is hoping to avoid a re-run of labour protests that rocked France for months last year under his Socialist predecessor Francois Hollande, which repeatedly descended into violence.
The president — whose personal ratings have slumped sharply since he came into office — is in the Caribbean on Tuesday visiting French islands hit by hurricane Irma last week.
Macron is determined to bring down France's unemployment rate — at 9.5 percent, roughly twice that of Britain or Germany — and sees simplifying the unwieldy labour code as key to achieving this.
Under his reforms, bosses would be given more freedom to negotiate working conditions directly with their employees rather than being subject to industry-wide agreements.
Compensation for unfair dismissal would also be capped — a move that has particularly angered unions, along with steps to make it easier for foreign-based companies to lay off staff in struggling French operations.
The CGT plans to follow Tuesday's actions with another protest day on September 21, with another two days later called by Jean-Luc Melenchon.
Recent polls show that only around 40 percent of French voters are satisfied with Macron's performance, with analysts putting the disappointment down to a combination of gaffes and poor communication.