A tidal surge in the North Sea has sparked severe flood warnings and evacuations on England's east coast.
Residents have been warned to watch the weather and tides
Prime Minister Gordon Brown has held an emergency Cobra committee meeting and the Environment Agency has warned of "extreme danger to life and property".
Norfolk and Suffolk have eight severe flood warnings. Parts of Essex, Lincs, North Yorks and Kent are also on alert.
Norfolk Police have visited 7,500 homes in Great Yarmouth to advise residents to leave the area.
They have been told to stay with family and friends outside the borough, or to move upstairs.
Environment Agency chief executive Baroness Young said some of the flood defences in Norfolk and Suffolk might not cope with the expected surge.
"I hope our defences can cope but this is a pretty severe weather event and some of them may not.
"And indeed on our current predictions some of them may be overtopped - even if they do stand up to the wave and wind activity."
'Wait and see'
She also suggested that people make checks on elderly relatives and neighbours.
In London the Thames Barrier was closed late on Thursday and tidal flood risk manager Andy Batchelor said it would contain the water.
"The high water here is at 1am and the tide will gradually build up... but be contained by the barrier and the walls that lead all the way out to the outer estuary - so London is safe," he said.
Speaking after the Cobra meeting, Environment Secretary Hilary Benn urged people in the affected areas to remain calm.
"We are doing all that we can but we are just going to have to wait and see what happens as the surge makes its way down," he said.
Norfolk Police, Norfolk County Council and Great Yarmouth Borough Council evacuated people from care homes and hospitals.
FLOOD ALERTS EXPLAINED
Severe Flood Warning - Severe flooding is expected. There is extreme danger to life and property. Act now!
Flood Warning - Flooding of homes and businesses is expected. Act now!
Flood Watch - Flooding of low lying land and roads is expected. Be aware, be prepared, watch out!
Source: Environment Agency
Two rest centres have been set up in the county, as more people could be brought in during the night.
Residents in low-lying areas of the Suffolk coast were advised to leave their homes as up to 1,300 properties could be affected there.
A Suffolk Police spokeswoman said: "Those most likely to be vulnerable to flooding should leave their properties and seek shelter with friends and family outside the affected area if possible."
Leisure centres in Lowestoft and Leiston, as well as Beccles Public Hall, are being used as rest centres.
Meanwhile, in Great Yarmouth, Norfolk, sandbags were being prepared for homes and businesses.
The flood alerts are a response to weather and tide patterns being tracked this week by the Environment Agency and the Met Office.
Wind speeds exceeding 50mph (80km/h) are predicted, with the storm surge expected to peak at about 0430 GMT around Immingham, near Grimsby, and then at 0700 GMT on the East Anglian coast.
In a brief Commons statement on Thursday afternoon, Mr Benn said: "A tidal surge of up to 3m [10ft] is making its way down the North Sea which could coincide with peak high tides.
"There is a risk of flood defences being over-topped on the coast and in tidal rivers, especially in East Anglia, particularly the Norfolk Broads and the coast south of Great Yarmouth including Lowestoft, and areas south of this as far as the coast of Kent."
BBC weather forecaster Carol Kirkwood said "particularly high tides" were exacerbating the situation.
There are eight severe flood warnings, 10 flood warnings and 24 flood watches nationally, covering North Yorkshire, Lincolnshire, Norfolk, Suffolk, Essex and the north Kent coast.
The severe warnings in place from Great Yarmouth down to Shingle Street, and on parts of the River Bure, River Yare and River Waveney, carry an Environment Agency alert stating: "Severe flooding is expected. There is extreme danger to life and property. Act now."
From the north Kent coast around to Sandwich and Deal, people were being asked to monitor the weather and tides.
The impact there was expected to be less severe - with predicted tides 1.5m (5ft) above average - but there is still a risk of localised flooding.
The Environment Agency, Met Office, emergency services and council planners will be constantly assessing the tidal and storm situation in all the affected areas.
Large parts of Lincolnshire, Norfolk, Suffolk, Essex and Kent were left under water in 1953, and 307 people died, when high tides and a storm saw a tidal surge of 3.2m (10ft 6in).
The Environment Agency said it was "much better prepared now", but the Met Office added that it was predicting the highest tide since 1983 in Felixstowe, Suffolk.
For more information on flood risks call the Environment Agency's Floodline on 08459 881188.