Thousands of Peruvians marched in at least 11 cities Thursday night to protest against a new law that strips workers between ages 18 and 24 of some labor rights, and some of the demonstrations turned violent.

In Lima, some of the more than 5,000 marchers clashed with tear gas-hurling police, pelting them with sticks and stones and setting fire to trash in street bins.

Sixteen police officers were injured, one seriously, and 20 people were arrested in the capital, Interior Minister Daniel Urresti told reporters. There was no immediate word on civilian injuries.

The march was the fourth against the law since Congress passed it in early December with minimal debate. Critics say it favors employers, converting one in 15 Peruvians into cheap labor.

Under the law, workers ages 18 to 24 who are dismissed will not get severance pay. Workers in the age group also will now be entitled to only half the vacation time given to older employees.

And young workers won't be paid the twice yearly bonuses of one month's wages standard for others.