South Koreans were still in shock today after an overnight fire destroyed a 610-year-old wooden gate and the country's main national treasure.

Namdaemun, or "Great South Gate", collapsed after a five-hour battle by firefighters to save the monument.

"It's devastating, the pride of the nation has collapsed," Kim Cheol-su, 55, told the Yonhap News Agency.

"It's so bad this kind of thing happened on the first week day of the Lunar New Year."

Police have not determined what caused the fire, Sohn Young-jin, a police official, said on Monday.

Police said they were searching for a man suspected of breaking into the stone and wooden structure and starting the fire.

"From what we can see, except for the pillars on the first floor, the gate has been completely burnt down," said a fire official.

The wooden pavilion on top of the gate's stone base was reduced to a smouldering hulk by the fire.