More tankers were blocked from the largest French oil port of Fos-Lavera as workers extended a strike that began on Thursday.
The three-day work stoppage has stopped some 29 oil tankers from entering the port, or from being loaded or unloaded, according to authorities.
“Some 57 ships are blocked at sea or at quay, including 29 that carry oil,” authorities said in a statement.
France's main union for port workers, part of the Confédération Générale du Travail (CGT), last month called for weekly 24-hour strikes to protest against government reforms and proposed privatization.
Normal operations at the Fos-Lavera terminals, part of the state-run Port Autonome de Marseilles, have been disrupted and tankers have had to wait for berthing.
Strikers intensified their protests last week, extending their 24-hour strike on Thursday to Friday, and then to Monday.
“We are in the process of hardening the action,” Pascal Galeote, head of the Marseille port CGT union, told Reuters.
“We are calling for partial strikes on Tuesday and Wednesday and for full-on strikes at least on Thursday and Friday.”