South Korea's coronavirus infections are back "in full swing" and spreading nationwide after members of a church attended a political demonstration, authorities said on Thursday, threatening one of the world's Covid-19 success stories.

The Korea Centers for Disease Control and Prevention (KCDC) reported 288 new cases as of midnight on Wednesday, marking a week of triple-digit daily increases, although down slightly from the previous day's 297.

"This is a grave situation that could possibly lead to...