Residents in parts of the county awoke to find their homes and businesses several feet under muddy water.
Heavy rains and gale force winds brought misery to St Austell, Lostwithiel, St Blazey, Bodmin, Par, Mevagissey and Luxulyan.
There were no reports of serious injuries but police declared the flooding a "major incident" with scores of residents evacuated, schools closed, the transport network hugely disrupted and train services stopped by a landslide at Lostwithiel.
Eco-attraction the Eden Project, near St Austell, also closed until Saturday at the earliest, it said.
The Prime Minister said the region suffered a "very difficult night".
"I know that everyone is working around the clock to get this sorted," he said during Prime Minister's Questions.
"We have said we stand ready to help in any way we can.
"We have to remember when the flood waters actually start to recede, that is when many of the biggest problems begin, over insurance and getting people back into their homes, and we've got to make sure we help people in every way we can."
The region was hit on the first anniversary of the floods that devastated Cumbria.
The most high profile victim was Pc Bill Barker.
The 44-year-old was guarding the Northside Bridge in Workington on the eve of his birthday when it collapsed, throwing him into the River Derwent.
His body was later discovered washed up on a beach.
Today's rescue and clean up operations evoked memories of the 2004 floods in the north Cornwall village of Boscastle.
Barry Green, owner of Lostwithiel Bakery, was at work early this morning when the River Fowey burst its banks.
The 46-year-old said his premises on Quay Street were "pretty much destroyed".
"It all kicked off at about 5.15am when I heard a bubbling outside in the drains," he said.
"I was up at 3am and everything was fine.
"I had to quickly get the stock high up off the floor and call the flood lines to let them know we had a problem.
"Within three or four minutes it was knee high.
"One minute it was just lapping at the front door with no problem and I was just lifting a few boxes then I opened the front door and it just came pouring through.
"The speed of it was very, very surprising.
"One minute I was making bread the next I was wading waist high through muddy water.
"We've had people coming in to help but it's pretty much destroyed the shop."
Mr Green left the shop and drove away from the bakery - located at the lowest point of the valley - to safety.
"I managed to get the car out just in time," he said.
"It was like driving up a river."
Gillian Avery, a St Austell Bay guest house manager, said she had not seen flooding like it since the 1990s.
She told Sky News: "I woke up and stepped into 2ft of filthy muddy water in my bedroom, which is on the ground floor."
An Environment Agency spokesman said: "This is a serious incident and our thoughts are with those people whose homes have flooded.
"The Environment Agency has deployed teams to check river flood defences and to assist emergency services with the recovery process.
"The joint Environment Agency/Met Office Flood Forecasting Centre issued an extreme rainfall alert yesterday afternoon to give emergency responders and local authorities advance warning of the heavy rain overnight, which has mainly resulted in flooding from drains and surface water run-off."
As well as police and fire crews, coastguard rescue teams helped with the search missions along with RAF helicopters
Meteorologists said Cardinham, on Bodmin, recorded 18.8mm of rainfall in one hour and 50mm in nine hours.
The AA - which used specialist water rescue Land Rovers to help stricken motorists - said around 25 vehicles were recovered when drivers attempted to pass down flooded roads or were caught out by the river bursting its banks.
Chief Superintendent Chris Boarland, commander for Cornwall, said: "A major incident was declared this morning due to the severe weather which has affected many parts of Cornwall.
"While the weather is improving the rising river levels pose a serious danger to the public.
"People should avoid travelling throughout the county and must never try to cross flooded roads.
"The police continue to work with our partners to monitor the latest weather and flooding forecasts and we will act as needed."
In Lostwithiel - the worst affected area - the Rivey Fowey had burst it banks.
In a riverside car park motorists had left their vehicles, abandoned in a foot of water.
Inch-deep mud and debris lay strewn across streets in the centre of the town.
Firemen were helping residents stranded by the flood water and road sweepers and other heavy machinery had been brought in to help with the clean up.
Many residents had used sandbags to try to keep their homes and businesses safe.
Darron Burness, of the AA's special operations response team, warned that many roads in the area could still be dangerous to motorists.
"Lostwithiel is the worst-affected area but many other roads in the area are closed or impassable," he said.
"There's a lot of debris on the roads, particularly stuff washed down from old quarries, and many roads are covered in thick mud. "We are concerned that come nightfall, it will be even more treacherous, as it will be difficult to spot debris, standing water and other hazards on the road.
"People in the affected areas should stay put in a place of safety, keep tuned to the news reports and heed any police advice about when it is safe to travel.
"Unfortunately, some people will take a chance but if they get stuck, they're not only putting themselves at risk but potentially others too."
Devon and Cornwall Police said the A390 remained closed and the old bridge in Lostwithiel was closed to both traffic and pedestrians.
The rest centre at Polkyth remains open and residents in 100 homes in Lostwithiel have been offered the opportunity to go to the centre.
"The water has all but disappeared in the flooded areas in Mevagissey but Station Road in St Blazey remains impassable due to flood water," a spokesman said.
"Flood warnings remain on the Rivers Kensey, Fowey and Neat."
Stephen Ellison, a meteorologist with MeteoGroup UK, the weather division of the Press Association, said there would be more rain in Cornwall overnight but nothing like last night.
"It will be a mainly fine early evening with clear spells and a few heavy showers," he said.
"Cloud will then increase from the west bringing more frequent showers or longer spells of rain throughout much of the night, some heavy, but nothing on the scale of last night.
"Tomorrow there will be sunny intervals and showers, some heavy, with the chance of thunder, especially around northern coasts."
VIEW GALLERY
Comments