Italy’s prime minister has said at least 16 million people are in mandatory quarantine in Lombardy region and also in 14 provinces.
The lock-down will last until early April.
The dramatic escalation in the country’s efforts to contain the new coronavirus will close gyms, pools, museums and ski resorts.
Italy is Europe’s worst-hit country and reported a steep rise in virus infections on Saturday.
The new measures, which also apply to financial centre Milan and tourist hotspot Venice will last until 3 April.
The death toll in Italy has passed 230, with officials reporting more than 50 deaths in 24 hours. The number of confirmed cases jumped by more than 1,200 to 5,883 on Saturday.
The whole northern region of Lombardy, home to 10 million people and Italy’s financial centre Milan, will be closed off except to emergency access, and 14 provinces including Venice, Parma and Modena will be subject to the same measures – affecting a total of around 16 million people.
Prime Minister Giuseppe Conte said the provinces affected were Modena, Parma, Piacenza, Reggio Emilia, Rimini, Pesaro and Urbino, Alessandria, Asti, Novara, Verbano Cusio Ossola, Vercelli, Padua, Treviso and Venice.
Until now only around 50,000 people in northern Italy had been affected by quarantine measures.
Night clubs, gyms, swimming pools, museums and ski resorts will be closed.
Restaurants and cafes can open but customers must sit at least a metre apart.
People will be told to stay home as much as possible, and those who break the quarantine could face three months in jail.
Sports competitions will close to the public, and the president of Italy’s football players’ union has called for all matches to be postponed.
The World Health Organization has advised Italy to strongly focus on virus containment measures as infections spread in the country.