Hundreds were forced to evacuate and power was cut, according to authorities.
100mm of rain per hour
The southern part of Seoul saw more than 100mm of rain per hour on late Monday, with some parts of the city receiving 141.5mm of rain per hour, the heaviest rainfall in 80 years, according to local media citing Korea Meteorological Administration (KMA).
This rate was also the highest since authorities began keeping records.
Since midnight Monday, parts of Seoul saw a total of 422mm of rain, prompting authorities to raise the highest Level 3 emergency alert.
Reuters reported that at least five people died in Seoul and three others in the neighbouring Gyeonggi Province by early Tuesday, the Central Disaster and Safety Countermeasures Headquarters said.
In total, four died after being trapped in flooded buildings, one was believed to have been electrocuted, another person was found under the wreckage of a bus stop, and another two in a landslide, it said.
Nine others were injured and at least seven people remain missing, the ministry said.
More heavy rain expected
More heavy rain are forecast to move through Seoul until Thursday, Aug. 11, which could bring additional flash flooding to the region.
People were already seen in photos wading across roads up to their thighs in water in the city on Monday.
Drains backed up in some parts of Seoul, which sent water pouring back into streets and subway stations, according to the Seoul Metro.
A number of subway stations were closed due to flooding.
Lines were temporarily suspended on Monday night, as authorities worked to reopen the stations on Tuesday morning.
In the glitzy, dense Gangnam district, some buildings were without power and had been inundated.
Cars, buses and subway stations there were submerged, leaving people stranded, Reuters reported.
Data on damages
Data showed at least 765 facilities were damaged, while about 52 highways and roads have been blocked.
There were about 391 victims across the greater Seoul area, most of whom had to stay at local schools and gyms, while another 399 temporarily evacuated to community centres and schools.
President Yoon Suk-yeol presided over an emergency response meeting, ordering authorities to focus on preventing casualties and quickly controlling and recovering flooded areas, the disaster headquarters said.
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