Hospitals in London were forced to ask patients to stay away after their emergency departments were flooded as thunderstorms lashed the capital on Sunday.
East London's Whipps Cross and Newham hospitals urged patients to look elsewhere for urgent care as they worked to fix the problem.
Residents also used buckets and wooden boards to to create makeshift flood defences for their homes.
London fire brigade said on Monday it had received about 600 calls for help as it responded to reports of collapsed ceilings and vehicles trapped in floodwater.
Torrential rain fell fell about 2pm on Sunday, with St James's Park in central London recording 41.6mm, making it the wettest part of the country.
The Environment Agency has six active flood warnings in place across south-east England.
Newham Hospital said on Sunday: "We're still here if you need us but to help us while we fix things, please attend a neighbouring hospital if possible."
Whipps Cross hospital in Leytonstone issued a similar appeal, saying it was experiencing "operational issues due to the heavy rainfall".
Tube stations were also flooded, with social media videos showing Pudding Mill Lane in east London under what appeared to be at least a foot of water.
Eddie Elliott, 28, said the flooding was the worst he had ever seen it, after he cycled past Queenstown Road station where the road had been “totally shut down”.
"Having been born and raised in London, I have never seen anything quite like it," he told PA.
“It stands out as the worst I’ve experienced personally … totally shut down the whole road with buses stood broken down in the water.”
Chris Date tweeted a photo of a bus in Bakers Arms, Walthamstow, saying the flood water was "above the kerb".
"It's impossible to walk on the pavement. To get on that bus the water came up to my shins. This is a canal, not a road," he said.
Buses and cars were abandoned on flooded roads, with social media users posting videos of partially submerged vehicles in south-west London.
The Met Office earlier issued warnings of lightning strikes and flooding, predicting almost double the monthly average rainfall for July.
Police closed a road near Queenstown Road railway station in south-west London, where three double-decker buses became stuck under a bridge.
A bus driver said passengers had to get off his vehicle after it started taking on water.
Other motorists in Walthamstow, north-east London, abandoned their vehicles as the rain hammered down.
London's Metropolitan Police urged people not to attempt to drive through flood water.
“There’s is quite a lot of flooding in London tonight,” they said.
“If you see deep water, please do not try to get through. Rescuing people uses a lot of resources, and so it’s wiser to apply good judgement and find a different route.”