More than 3500 people have tested positive for the coronavirus in Japan and 85 have died - not a huge outbreak compared with some hot spots, but the numbers keep rising with particular alarm over the spread in Tokyo, which has more than 1000 cases, including 83 new ones on Monday.

Prime Minister Shinzo Abe made the announcement on the state of emergency in remarks to media televised live.

An emergency, which Abe said would last about a month, will give governors authority to call on people to stay at home and businesses to close, but not to order the kind of lockdowns seen in other countries.

In most cases, there are no penalties for ignoring requests, and enforcement will rely more on peer pressure and respect for authority.

Pressure had been mounting on the government to take the step although Abe has voiced concern about being too hasty, given the restrictions on movement and businesses that would ensue.

The stimulus package of hundreds of billions of dollars is due to be rolled out this week.

An emergency would appear to have public support. In a poll published on Monday by JNN, run by broadcaster TBS, 80 per cent of those surveyed said Abe should declare it while 12 per cent said it was not necessary. His approval rating fell by 5.7 points from last month to 43.2 per cent, the survey showed.

Kenji Shibuya, director of the Institute for Public Health at King's College, London, said the emergency was too late given the explosive increase in cases in Tokyo.

"It should have been declared by April 1 at the latest," he said.

Sounding alarm over the high rate of cases that could not be traced, Tokyo Governor Yuriko Koike indicated last week that she would favour a state of emergency as a way to help her urge residents to adhere to stronger social-distancing measures.

An expert on the government's coronavirus panel said Japan could avoid an explosive rise by reducing person-to-person contact by 80 per cent.

Under a law revised in March to cover the coronavirus, the prime minister can declare a state of emergency if the disease poses a "grave danger" to lives and if its rapid spread could have a big impact on the economy.

While Japan's coronavirus epidemic is dwarfed by the 335,000 infections and more than 9500 deaths in the United States alone, experts worry a sudden surge could overwhelm Japan's medical system.

Abe must seek formal advice from a panel of experts before deciding to go ahead and declare the emergency.

Governors in Tokyo and elsewhere have asked citizens to stay home on weekends, avoid crowds and evening outings, and work from home. That has had some effect, but not as much as many experts said was needed.

Australian Associated Press