A tornado ripped roofs off homes and tore down walls in a residential area of northwest London today, injuring six people.
Television footage showed a trail of destruction in Kensal Rise, with trees uprooted and cars damaged by falling debris.
Hundreds of people have been displaced from their homes.
Tornadoes in Britain are normally weak and rarely cause damage. Local resident Daniel Bidgood was in his house when the tornado, which he said was about 20 metres across, smashed his windows.
"It was very large and certainly very powerful," he told BBC television.
"You could see it ripping up heavy chunks of mortar and smashing it into cars." One man in his 50's was taken to hospital with head injuries.
Five other adults were treated for shock and minor injuries after the tornado struck at around 11am.
Around 100 houses were damaged, a Fire Brigade spokesman said, "so I would imagine several hundred people may be displaced".
Some of those evacuated were being housed in local churches. More than 20 fire engines were sent to the scene and the area was cordoned off.
The London tornado was rated at T4 on a scale of 0 to 10. This means winds were moving at between 115 and 136 mph, rating the tornado as severe, said a spokesman for the Meteorological Office.
Britain experiences between 30 and 40 tornadoes in an average year, he said, but they rarely hit built-up areas.
The British Met Office issued severe weather warnings for southern England, with heavy rains and winds gusting up to 70 miles per hour, after the warmest autumn in the last 347 years.