Around 50 cars were destroyed by the extremists in the police car park in Grenoble as president Macron is due to adopt the reforms by executive order tomorrow.
One police officer is in hospital after he inhaled smoke from the arson.
The fire began as unions staged a second day of protests against Mr Macron’s controversial reforms.
An anarchist group called Révolte Anarchiste des Gendarmes Exilés claimed responsibility for the fire.
Organisers claim up to 55,000 people attended protests in Paris today.
In cities across France, demonstrators waved anti-capitalist messaged against President Macron.
The newly-elected president wants new reforms to be passed through allowing bosses to be able to hire and fire employees more easily, which have made French people fear employees’ working conditions will get worse.
The President has said that the reforms are needed to power the stagnant French economy and boost jobs.
The Communist-backed CGT union organised Thursday’s protests and its secretary general, Philippe Martinez, called the first round last Tuesday “a good start”.
Eight people are currently standing trial this week in Paris for setting fire to a police car in May lat year during protests against labour reforms.
The anarchist group said: “Whatever the outcome of this trial, we will continue to act against police and justice. Our hostility is fire that is spreading.”
President Macron has vowed to continue with his labour reforms even though 53 per cent of the French back the protests.
Left-wing leader Jean-Luc Melenchon has called for more street protests on Saturday.
He said: “This is only the beginning”.
Last week the CGT organised more than 180 marches and 4,000 strikes nationwide and urged rail worker,s students and civil servants to join the demonstrations.
The International Monetary Fund has claimed that Labour reforms are crucial for France’s prospects.