In the latest protest in Lyon, police used tear gas against protesters who were setting garbage bins on fire.
The protests have turned violent in Paris and other cities, with clashes between protesters and police, and several hundred black-clad radical demonstrators breaking windows and destroying street furniture.
The hard-left CGT union reported that around 800,000 people marched in the capital, the highest daily number since the start of the protest movement.
The government's decision to bypass a vote in parliament to impose the reform has triggered the biggest domestic crisis of President Emmanuel Macron's second term.
Despite the protests, Macron refused to back down on the reform in a TV interview on Wednesday, giving new momentum to the demonstrations.
Nightly protests have continued across France, with young people coordinating their actions on encrypted messaging services.
The unrest is threatening to cast a shadow over the visit to France next week of King Charles III, the first foreign visit he has made as monarch.