France put its 67 million people under lockdown in an unprecedented act during peacetime, and said it was ready to nationalise big companies suffering financial turmoil created by the deepening coronavirus crisis.
Drafting in the army to help transport gravely ill people and ordering border controls, President Emmanuel Macron said citizens could leave their homes only to buy food, go to work, seek medical care or get some exercise on their own.
In Paris, police patrolled eerily quiet boulevards, stopping drivers and pedestrians and demanding that they present an interior ministry document justifying their movement.
The form can be downloaded on the ministry website and citizens can present an electronic version on their smartphones.
"It's like the holidays in August," said flat caretaker Slavjanka Stankovic. "There's only pigeons and police out there on the Place de la Republique!"
France follows neighbouring Italy and Spain in introducing drastic curbs on public life in response to the pandemic.
France's death toll from the coronavirus has reached 148 and a total of 6,600 people have been confirmed infected.
Macron's government is deploying more than 100,000 police nationwide to enforce the restrictions.
Those who breach the conditions will be fined, although police were showing leniency in the first hours of the lockdown.
Prime Minister Edouard Philippe said there were plans to increase the fine for violations to 135 euros, from 30 euros now.
Hours before the lockdown took effect, Parisians rushed in large numbers to train stations or took to the roads to escape the capital.
Those who stayed behind descended on supermarkets and pharmacies, even though these were due to remain open.
The Paris exodus drew dismay from some in provincial France, where many fear city-dwellers will bring the virus with them and accelerate its spread.
As in many other European countries including Italy, Germany and Spain, schools are closed, restaurants and bars shuttered and factories idle.
Finance Minister Bruno Le Maire said France might nationalise big companies reeling from the market fallout.
He said he would mobilise 45 billion euros in emergency measures to help firms weather the storm, money that comes on top of 300 billion euros in government loan guarantees.
Le Maire said the unexpected strain on public finances meant he would have to tear up his 2020 budget.
The lockdown will last at least two weeks, Macron said.
Australian Associated Press