At 7:38 a.m., at the beginning of the morning rush hour, a span on the north side of the Seongsu Bridge suddenly gave way into the Han River below. Survivors of the collapse compared the sound of the collapse to thunder. Traffic was flowing at a speed of , and a total of six vehicles were involved in the incident, including a police van and a commuter bus carrying 31 occupants. The police van carried 11 police and auxiliary police officers, that were selected as model officers for celebrations that day. Heavy rain the night before the collapse reportedly reduced the number of vehicles on the road at the time. The collapse sent two cars and the police van down with the bridge, while an additional two cars fell into the river. The Number 16 bus operated by the was traveling North from Seoul Grand Park to Beon-dong and had nearly crossed the span when the bridge collapsed. The collapse caused the rear of the bus to tilt over backwards and plummet onto its roof, crumpling to half its size.
