Unions held a short march around the Bastille plaza to protest government plans to make working weeks longer and redundancies easier.
Similar protests against the legislation in recent months degenerated into clashes between riot police and radicals.
Police threatened to ban yesterday’s march, but the government agreed to let it proceed in a restricted area.
Police were systematically checking the bags of protesters before letting them enter the area.
Johanna Primevert, Paris police’s spokeswoman, told BFM television that 18 people had been arrested, most of them for possession of objects that could be used as projectiles.
Authorities ordered the removal of some street furniture and protection of bus shelters to avoid potential damage. Some of the boats docked in a port between the Seine and the Bastille plaza had been moved.
Police said some union members participated in and cheered recent violence. But the head of the CGT union, Philippe Martinez, insisted the organisation was not an accomplice of the troublemakers and condemned them.
Other protests were organised for yesterday in major cities to oppose the government’s labour legislation.