A city north of Tokyo has declared a state of emergency and two million people in its surrounds have been told to evacuate, as vicious floods caused chaos in eastern Japan.

Dramatic television footage captured a wall of muddy water gushing from the swollen Kinugawa river in Joso city, east-central Japan, which is home to around 65,000 people.

Earlier on Thursday, authorities issued pre-dawn warnings of unusually harsh rainfall to five million people.

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Typhoon Etau smashed Japan on Wednesday before moving out to sea, but trailing walls of rain continued to lash the country.

A person is winched to safety from their home hit by flooding.

“This is a scale of downpour that we have not experienced before. Grave danger could be imminent,” forecaster Takuya Deshimaru told an emergency press conference.

In Ibaraki prefecture, desperate residents waved towels at rescuers as they stood on second-floor balconies waiting for help.

“Please continue to ask for help. Please do not give up hope,” an NHK broadcaster said in an apparent message to helpless residents.

Prime Minister Shinzo Abe said the government was on high alert.

“The government will stand united and do its best to deal with the disaster … by putting its highest priority on people’s lives,” he said.

Rainfall reached 600 millimetres in the area around Joso, with weather officials expecting at least 200mm more in parts of eastern Japan.

This area included Fukushima, the site of the nuclear reactor crippled in 2011, before the downpour stops on Friday.

The torrential downpour exacerbated the contaminated water problem at the Fukushima nuclear plant as it overwhelmed the site’s drainage pumps.

It sent hundreds of tonnes of contaminated water flowing into the ocean.

Two people are missing, one feared stuck in a home that flood waters swept away. Up to 12 others are injured, including one lady with a broken leg. 

The deluge has been captured in a series of stunning images and video.

– with ABC and Getty Images