Police said about 50 anarchists were protesting Thursday's arrest of three people alleged to have sent a letter bomb to the La Stampa newspaper Tuesday, Italy's ANSA news agency reported.
The bomb failed to detonate.
An editor, photographer and driver for the Turin-based newspaper were trapped in their car as the protesters threw stones. The driver was hit in the wrist and arm.
Two groups took responsibility for the letter bomb in a letter issued before the protest.
The Informal Anarchist Federation and the International Revolutionary Front said the bomb was "dedicated" to two of the group's members arrested in September 2012 for allegedly kneecapping the chief executive officer of a Genoa power company in May.
"La Stampa is no more than a paper of the regime, so it is a target of our war against the state and society that sustains and legitimizes its existence every day," the federation said in the letter.