Torrential rains have triggered floods and landslides in southwest Japan, killing at least two people and prompting authorities to order more than 900,000 people to leave their homes, while another million were advised to move to safety, reports said.
The country's weather agency raised the alert to its highest level on Wednesday for parts of the southern island of Kyushu, the third largest of Japan's five main islands.
The emergency warning is issued "if there is a significant likelihood of catastrophes".
Officials confirmed two deaths to the AFP news agency, one in western Saga prefecture where a man was found in a car that had been swept away.
A second man died in Fukuoka as he tried to escape from a car trapped in rising floodwaters.
More than twice the usual rainfall for the whole of August has fallen over parts of Kyushu over the past 48 hours, washing away roads, causing rivers to burst their banks and forcing the suspension of train services.
Government spokesman Yoshihide Suga said a third person in Saga was in a state of "cardiorespiratory arrest".
"There are many reports of damage in different areas due to flooding of rivers, landslides and submerged houses, and there is a possibility of serious damage occurring in the coming hours," Suga added.
Weather agency official Yasushi Kajiwara said the areas issued special warnings were experiencing "unprecedented levels of heavy rains".
"It is a situation where you should do your best to protect your lives," he said.
Kajiwara also urged those living in areas currently under evacuation advisories to act quickly. "Please don't wait," he warned.
More than 900,000 people were ordered to evacuate, national broadcaster NHK said, while Japan's Fire and Disaster Management Agency said just over a million others were advised to leave their homes.
The agency said it received multiple reports of flooded houses in Saga and Nagasaki prefectures.
Evacuation orders and advisories issued by local authorities are not mandatory, although officials urge residents to heed them.
The Ground Self Defense Force - Japan's military - said it had deployed about 100 troops for disaster relief after a request from Saga prefecture. Public transport and businesses were affected.
At least 155 people died in July last year when torrential rains swept across western Japan, including the prefectures of Hiroshima and Okayama, in the worst flood-related disaster in three decades.