The unions are protesting the government's plan to remove the job guarantees given to dockworkers to win their acceptance of a port reform program in the mid-1990s which lessened union control over the supply of labor.
Under the reform, private stevedores and terminal operators use their own labor, but must hire local port companies, which grew out of the disbanded union labor pools, to handle cargo surges.
The government wants terminal operators to be free to use outside firms since the current system is seen as giving the local port companies a monopoly over the supply of labor.
The current system is blamed for making Italian ports more expensive and less productive than their rivals in the Mediterranean as well as in northern Europe.