Hong Kong reimposed tough social distancing measures on Wednesday -- shuttering many businesses and making facemasks on public transport mandatory -- after a spike in coronavirus cases threatens to undo months of success against the disease.

The finance hub was one of the first places to be struck by the coronavirus when it emerged from central China.

But it had impressive success in tackling the disease with just over 1,500 infections, eight deaths and local transmissions all but ended.

In the last two weeks, however, more than 200 local infections have been recorded and officials believe the virus is spreading again among the densely populated city of 7.5 million.

Health authorities described the situation as "very worrying", with over 70 of the recent local cases having an unknown origin.

Hong Kongers widely adopted facemasks to deal with the epidemic when it first emerged, but from Wednesday they were made compulsory on public transport.

A 70-year-old man was the first to be arrested for not wearing a mask on the city's subway under the new regulations, local newspaper Apple Daily reported.

Hong Kongers have widely adopted masks but they are now compulsory on public transport Hong Kongers have widely adopted masks but they are now compulsory on public transport Photo: AFP / Anthony WALLACE

Those who breach the rules face a HK$5,000 ($650) fine.

"It's a bit too late. It should be regulated at an earlier stage, as it would be safer," a bus driver told local news outlet RTHK.

Other restrictions also applied from Wednesday to many businesses in a city already mired in recession.

Public gatherings of more than four people are now banned, while several businesses -- including gyms, salons, bars, nightclubs and karaoke lounges -- were ordered to close.

Restaurants can seat customers for breakfast and lunch -- with a maximum of four people per table -- but can only serve takeaway food from 6:00 pm to 5:00 am.

"We don't know how the customers will react to the latest restriction yet as many of them might not even want to come out to get take-outs," Ah Long, manager of a restaurant in North Point, told AFP.

The current restrictions will be reviewed every seven days.