A man has died as Storm Desmond tore through Britain, bringing strong winds and heavy rain which caused Cumbria to declare a major incident.
The Environment Agency declared 130 flood warnings, while residents in some areas were evacuated from their homes.
Rain continued to fall overnight in Scotland, northern England and northern parts of Wales, and is likely to continue for a few hours yet, forecasters said.
The deluge left streets lined with terraced houses looking more like rivers as rescue teams set off in rubber dinghies to help stranded locals.
Bridges collapsed, rivers burst their banks and landslides were triggered as torrential rain swept through large swathes of the north of England and Scotland.
Prime Minister David Cameron said on Twitter: "My thoughts are with all affected by Storm Desmond. Teams are working to ensure swift response and help for those who need it."
Cumbria was the among the worst affected by the onslaught, and British Red Cross teams set up rest centres in Keswick, Appleby and Kendal, while medical groups issued an urgent call to draft in extra doctors amid fears the storm could cause casualties.
There was flooding in Carlisle and there were power cuts in many areas.
The 90-year-old man who lost his life is believed to have been blown into the side of a moving bus by a gust of wind, near Finchley Central station, north London, a Scotland Yard spokesman said.
Adrian Holme, from Cumbria Fire and Rescue Service, told the BBC the flood was "unprecedented" and "exceptionally challenging".
He said it was "absolutely devastating", adding: "The flood defences that were built here in 2012 haven't been breached, they have been over-topped. We have had 24 hours of constant rain."
He added: "This is absolutely devastating for the town of Keswick. As you can see behind us, the water is huge and there are hundreds of properties that have been devastated and flooded. And some of these people have been flooded three times, our hearts must go out to them."
It is believed that more than 100 people were evacuated from the town.
Flooding and landslides also brought disruption to Scotland, and homes were evacuated as rivers burst their banks in the Borders and Tayside.
And hundreds of homes in Wales were left without power as strong winds battered power lines.
Electricity North West said at 3.15am that its engineers were continuing to work with emergency services in response to large power outages across the region.
Some 55,000 properties in Lancaster, Morecambe, Carnforth and the surrounding area are without power following a breach of flood defences at a the main substation serving the area.
Generators were being delivered to key sites that were determined to be most in need but residents were being warned that power may not be restored to some areas for a number of days.
Another 4,000 properties were without power across Cumbria and new faults were still being identified because of further flooding.
Mark Williamson, operations director for Electricity North West, said: "We're continuing to work closely with the emergency services to coordinate the best response we can.
"We are prioritising generators for those most in need, including the respite centre at Salt Ayre Leisure Centre. We also have generators en route to Morecambe Fire Station to aid the emergency response.
"We are doing all we can and our engineers are continuing to work hard to restore power where it is safe to do so."
A fire and rescue crew had to save a member of the public who was found clinging to a tree after they had tried to reach a horse stranded in a flooded field in Northumberland before being swept away by the flood water.
The RSPCA had to call off their attempts to rescue the creature because of the strength of the flood.
MeteoGroup forecaster Gemma Plumb said that Shap in Cumbria had 171mm of rain in the 24 hours to 6pm on Saturday night - and another 60mm in the six hours afterwards.
She said weather should dry out later during the day - before more rain heads up from the south west on Sunday night.
Lancashire Fire and Rescue Service said the county's firefighters dealt with more than 300 calls for help in Lancaster and surrounding areas as flooding hit the Cable Street, Water Street, Chapel Street, Damside Street and St George's Quay parts of the city centre.
Crews from all over the Lancaster and Morecambe district attended a range of situations including helping cars stuck in flood water, affected electrics, flooding and road traffic collisions and two fires.
Crews were also involved in a major operation to pump away from the electricity sub-station.
Unfortunately the flood waters rose too quickly to be able to prevent a power failure that was now affecting much of the Lancaster, Carnforth and Morecambe areas.
The service has not escaped flood damage itself, as the fire station in Cable Street has been evacuated and four fire engines suffered damage from flood water.
Area manager Phil Cox said: "At one stage we were recalling staff from all over the county to help out. At times it was difficult responding to the volume of calls and although Lancaster has been badly affected, we are thankful that we have not had to deal with as much as colleagues in Cumbria have experienced."