The governor projects the virus will soon infect more than half of the 40 million people in his state.
California has issued a "stay at home" order to residents as it tries to stem the march of the coronavirus across the most populous US state.
Governor Gavin Newsom told Californians they should only leave their homes when necessary during the pandemic.
He earlier estimated more than half of the 40 million people in his state would contract Covid-19 in just the next two months.
The virus has so far claimed 200 lives in the US and infected 13,678 people.
Governor Newsom said: "This is a moment we need to make tough decisions. We need to recognise reality."
His order will allow residents to leave their homes to buy groceries or medicine, or walk a dog or take exercise, but seeks to limit public interactions.
About half of the state's population are already subject to similar stringent measures, including the city of San Francisco.