South Korean activists burned the North's flag Sunday near a theatre where Kim Jong Un's sister and the South's President were to watch a Northern musical display in the culmination of their Olympic rapprochement.
Some 140 members of Pyongyang's Samjiyon Orchestra were to give a concert in Seoul as part of a cross-border deal in which the isolated, nuclear-armed North sent hundreds of athletes, cheerleaders and others to the Pyeongchang Winter Games in the South.
Kim on Saturday invited President Moon Jae-in to a summit in the North, an offer extended by his sister and special envoy Kim Yo Jong, who made history as the first member of the North's ruling dynasty to visit the South since the Korean War.
But the rapprochement pushed by the dovish Moon has angered conservatives who accuse him of being a North Korea sympathiser.
"Having these red communists in the heart of Seoul is an utter humilation!" one shouted near the venue as dozens of others waved banners condemning both Moon and Kim Jong Un.
"We are against the ugly political Olympics!" read one banner.
Some set a North Korean flag on fire before police intervened, and others chanted "Let's tear Kim Jong Un to death!" as they ripped up posters bearing his portrait.
The North's presence has dominated the headlines in the early days of the Olympics, with all eyes turning to Swiss-educated Kim Yo Jong, believed to be 30, who is among her brother's closest confidantes.
The protesters accuse Moon of allowing North Korea to stage its "propaganda" in Seoul and undermining the military alliance with the US.
Sunday's concert -- the orchestra's second and final show -- was expected to feature South Korean pop songs as well as North Korean music, and to be watched by Moon, Kim Yo Jong, and Kim Yong Nam, the North's ceremonial head of state, who has become technically its highest-level official ever to visit the South.
Public interest in the show was huge, with nearly 120,000 people applying for just 1,000 tickets.
Civilian contact is strictly banned between the two Koreas, which have technically remained at war since the 1950-53 Korean War ended with an armistice instead of a peace treaty.
Tensions soared last year as the North staged a series of nuclear and missile tests in violation of UN resolutions, while leader Kim and US President Donald Trump traded colourful insults and threats of war.
Moon has long sought engagement with the North to bring it to the negotiating table, and for months has promoted Pyeongchang as a "peace Olympics".
But controversy over the North's participation -- particularly the formation of a unified women's ice hockey team, seen as unfairly denying Seoul's own citizens a chance to compete on the Olympic stage -- has hit his approval ratings.
Many older South Koreans on both sides of the political divide harbour a nostalgic longing for some form of reunification -- conservatives through the North's collapse, liberals through a more amicable arrangement.
But younger South Koreans -- many of whom voted for Moon in May -- have spent their adult lives in a culturally vibrant democracy regularly menaced by Pyongyang. They have far less interest in unification and fear its social and economic consequences.
A poll last year found almost 50 percent of over-60s believed the two Koreas can be reunified, while just 20.5 percent of those in their 20s agreed.