The nation reported 56 cases on Wednesday with 42 linked to the karaoke cluster, the health ministry said.
The first known case was a Vietnamese woman who sought medical help on Sunday, according to local media. Free COVID-19 testing has been offered to anyone who might have been exposed.
Health Minister Ong Ye Kung had earlier warned police would take action against violators.
“Any outlets providing hostess services, dice games and all this very close contact, were never allowed,” he told local media, according to broadcaster ChannelNewsAsia.
“So for this to now happen has been troubling (and) disappointing.”
Singapore police said in a statement they had arrested 20 women late on Wednesday, among them South Koreans, Malaysians, Thai and Vietnamese, for alleged vice activities at KTV lounges. The three operators of the lounges were being investigated for breaking coronavirus-distancing rules, but the arrests were not linked to the karaoke cluster, Today newspaper said.
Ong said there was no plan to reverse recently loosened restrictions because of the cluster, citing progress in vaccinations.
Among those infected was also a cruise passenger who was taken to hospital on Wednesday. Nearly 3,000 passengers and crew were confined to their cabins, awaiting tests as a result of the case.
Singapore was badly hit last year after COVID-19 ripped through crowded dormitories where the city’s thousands of foreign workers live but has dealt swiftly with most subsequent outbreaks. In May, it imposed specific restrictions aimed at slowing the spread of the Delta variant.
It is aiming to complete the vaccination of two-thirds of its population by August 9, the country’s national day.
