The Campania region, which includes Naples, announced that it will enter lockdown on Friday as the number of COVID-19 cases rises.
The demonstrators threw lighting bombs and other rockets at policemen, who responded with tear gas.
There were unconfirmed reports that protesters stormed local government offices.
Jacob Wondries posted a video of the unrest on Twitter with the following caption: “Large regional anti-government protests against the COVID-19 virus are now occurring in Naples, Italy.
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“The situation appears to be out of control. Thousands of people in the streets who fought with the police have stormed the regional government offices.”
Footage spread on social media showed thousands of protesters, many of them wearing masks to protect against the Corona virus, marching around the city.
Many were chanting “freedom, freedom, freedom” during their procession.
Savannah Radio, which was covering the protests, tweeted: “All Naples is in revolt!
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“Residents are protesting en masse against the lockdown.
“At 11:00 pm, the curfew was activated, and the images are live: the civil disobedience continues.”
They added later: “Protests are also underway in Salerno, Nocera and other cities in Campania.”
The number of COVID-19 cases in Italy has increased sevenfold since the beginning of October.
Vincenzo de Luca, president of the Campania region, has ordered a night-time curfew and schools close.
He posted on Facebook: “Current data on infection makes any kind of partial measures ineffective. It is necessary to close everything, except for those companies that produce and transport essential goods.
“We need to make one last effort to get things under control. We need to shut everything down for a month, for 40 days.”