The nation has been held up as a global model in how to curb the virus, but the order from the Seoul mayor on Saturday followed the new infection cluster in Itaewon, one of the city’s busiest nightlife districts.
More than two dozen cases were linked to a 29-year-old man who tested positive after spending time at five clubs and bars in Itaewon last weekend.
Health authorities have warned of a further spike in infections, with around 7,200 people estimated to have visited the five establishments identified.
“Carelessness can lead to an explosion in infections,” Seoul mayor Park Won-soon said, adding the order will remain in effect indefinitely.
Of the 18 new South Korean cases reported on Saturday, 17 were tied to the Itaewon cluster, according to the Korea Centers for Disease Control and Prevention.
The jump in new infections comes as everyday life in South Korea has slowly started returning to normal, with the government relaxing social distancing rules last Wednesday.
The nation endured one of the worst early outbreaks of the disease outside mainland China, and while it never imposed a compulsory lockdown, strict social distancing had been widely observed since March.
The country’s extensive “trace, test and treat” programme has drawn widespread praise, and the latest outbreak appears to be under control.
34 new cases were reported on Sunday, taking the total to 10,874 – the largest daily increase in a month.