Horrified commuters watched as a central span of the four-lane Songsu bridge plunged into the Han River, taking cars, vans and a fully-loaded bus down with it.
Witnesses said there was no warning before a 48-meter central section of the steel-girdered bridge sheared cleanly from the main structure and fell into the Han.
"I don't want to think of this nightmare again. How could a bridge which millions of citizens use every day collapse so easily?" said 38-year-old schoolteacher Kim Min-ja.
Kim, who teaches at an elementary school, was plucked to safety by rescue boats after the car in which she was travelling careered headlong into the void left by the collapse. She said she managed to wrestle her way out of the car and swim to safety.
President Kim Young-sam demanded a thorough investigation and heavy punishment for those responsible. He called an emergency cabinet meeting after which the government released a statement expressing its deep regret and shock.
Only last week questions were asked in parliament over the safety of the 17 bridges which span the Han in Seoul, and Construction Minister Kim Woo-suk said there was no cause for concern.
President Kim accepted the resignation of Seoul city mayor Lee Won-jong, the presidential spokesman said.
City officials said repairs had been scheduled on the bridge on Thursday evening but had been postponed owing to bad weather. The central span was not slated for repair, they added.
The Songsu was built by one of South Korea's biggest construction firms, Dong Ah Construction Industrial Co.
A Dong Ah official said the bridge had been designed for vehicles no heavier than 33 tons. But the bridge, one of the city's busiest, now allowed loads of more than 43 tons.
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