Italy's prime minister has said at least 16 million people are now under lock-down in Lombardy region and also in 14 provinces until early April.
The dramatic escalation in the country's efforts to contain the new coronavirus will close gyms, pools, museums and ski resorts.
Weddings and funerals are also suspended under the new measures.
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.
"We want to guarantee the health of our citizens. We understand that these measures will impose sacrifices, sometimes small and sometimes very big," Prime Minister Giuseppe Conte said early on Sunday.
"But this is a time where we must take responsibility for ourselves."
Coronavirus: Northern Italy quarantines 16 million people - BBC News