Philippine President Rodrigo Duterte has threatened a martial law-like crackdown to stop people flouting a virus lockdown in the nation's capital.

Duterte spoke a day after authorities reported an upsurge of cars on Manila's roads, which have been nearly deserted since a sweeping lockdown was imposed a month ago on about half the country's 110 million people.

"I'm just asking for a little discipline. If not, if you do not believe me, then the military and police will take over," Duterte said in a televised speech late Thursday.

"The military and police will enforce social distancing at curfew... It's like martial law. You choose," he added. 

Duterte has repeatedly threatened to impose nationwide military rule over the Philippines, where the mere words evoke the worst rights abuses of the Ferdinand Marcos dictatorship.

The closest Duterte has come is the imposition of martial law over Mindanao, the nation's southern third, in response to Islamic State-inspired militants' siege of the city of Marawi.

As reported cases of the new coronavirus started to climb in March, Duterte ordered a quarantine of the main northern island of Luzon, which includes the capital's 12 million people.

Only essential workers and people buying food or medicine are officially allowed outside their homes, yet many others have flouted the rules.

In some places, impoverished day labourers have no choice but to go out looking for work, though police have also reported breaking up social events like amateur boxing matches.

Police said they have warned or arrested tens of thousands of people caught outside their homes without valid reasons.

The Philippines has detected some 5,660 coronavirus cases, and recorded 362 deaths, but those figures are expected to climb as the nation ramps up testing.