By Issei Kato

JOSO, Japan (Reuters) - Unprecedented rain in Japan unleashed heavy floods on Friday that tore houses from their foundations, uprooted trees and forced more than 100,000 people from their homes.

Helicopters hovering over swirling, muddy waters rescued many people from the roofs of their homes. Seven people were missing and at least 17 were injured, one seriously.

Some areas received double the usual September rainfall in 48 hours after tropical storm Etau swept across Japan's main island of Honshu. In some places, rain-swollen rivers burst their banks.

A 63-year old woman was missing in a landslide that hit her home while a man in his 70s in the town of Joso, 56 km (35 miles) north of Tokyo, was feared trapped when water engulfed his home, NHK national television said.

"We heard a huge sound like a thunderclap, and then the hillside came down," a man told NHK, referring to the landslide that swept away his neighbour.

Television broadcast footage of helicopters winching people to safety, including an elderly couple clutching a pair of struggling dogs as the flood tore away pieces of their home.

A further 800,000 people were at one point advised to evacuate after officials issued predawn warnings of "once in a half century rains" to 5 million people in areas east and north of Tokyo.

Japan has put heavy emphasis on disaster prevention since a 2011 earthquake and tsunami killed nearly 20,000 people and authorities are keen to avoid the kind of criticism they faced then, for what was seen as a sluggish response.

The government set up an emergency centre, and Prime Minister Shinzo Abe told a meeting of ministers that the "unprecedented" rain had created an emergency.

"The government will work as one to prioritise the safety of the people and do our best to prevent any further disaster," Abe told reporters.

Television footage showed people in Joso waving towels while waiting for help on the upper floors of homes engulfed in floods after the overflowing Kinugawa river swept through.

"I thought I was safe because I live on a hill, but pretty soon the water came up and everything was washed away," a barefoot man told Fuji TV after his helicopter rescue.

Up to 12 military helicopters took part in the rescue along with an initial 55 members of Japan's military, the Self Defence Force. Officials said their number would rise.

Rescue workers rushed to find people before nightfall.

Part of a hotel in the town of Nikko, famed for its shrines and temples, had collapsed, Kyodo news agency said, but there were no reports of injuries.

Rainfall reached 500 mm (20 inches) around Joso, NHK said, with weather officials expecting at least 200 mm (8 inches) more in parts of eastern Japan, including Fukushima, the site of the nuclear reactor crippled in 2011, before the downpour stops on Friday.

(Additional reporting by Kaori Kaneko; Writing by Elaine Lies; Editing by Robert Birsel and Clarence Fernandez)