Homes were evacuated and people left stranded in their cars as lightning, hail, heavy rain and flash flooding hit east and south-east England.
The freak weather brought roads and railway services to a standstill in parts of Sussex, Essex and London.
More than half the average total rain for England in July fell in an hour in some areas, the Met Office said.
London Fire Brigade said it had to help people trapped in their cars across the capital.
Network Rail said most train services were returning to normal following earlier delays and cancellations caused by flooding and an electricity substation being struck by lightning.
However, services to Brighton are not stopping at Worthing because of flooding in the station subway.
In London, the Metropolitan Line is running with severe delays because of flooding.
Flights to and from Gatwick Airport were also delayed by the weather.
The town of Great Dunmow in Essex was inundated by 43mm (1.7in) of rain within an hour, while residents in Isfield, Sussex, were hit with 37mm (1.5in) of rain.
A road in the village of Thaxted, Essex collapsed and several homes were left flooded. Five people in the village were taken to safety by fire crews.
In north-west London, two women had to be rescued from a car stranded in flash floods outside South Ruislip station.
London Fire Brigade said it had to deal with two stranded cars on Eastcote Road in Harrow.
At one stage six cars were stuck in floodwater in Uxbridge, west London, according to the AA's Special Operations Response Team.
Fifty properties were also flooded on Bedford Road in Ruislip.
Jamie Davis, 17, of Ickenham, west London said the downpours, with flashing lightning and loud thunderclaps, began at 09.00 BST and lasted for at least 45 minutes.
"My back garden is completely flooded. The drains are bubbling up and we can't flush the toilet," he said.
In Sussex, Southern Water said torrential rain had caused sewers to be "overwhelmed by the sheer volume of water".
East Sussex Fire and Rescue Service received about 300 calls within a few hours, while a further 60 calls were passed on to Cambridgeshire Fire and Rescue Service as part of emergency resilience arrangements.
It said its crews had dealt with a large number of weather-related emergency calls which had included flooding, lightning strikes "and false alarms caused by the weather".
Sussex residents said the storm there started at 05:30 BST with hailstorms particularly heavy in the region.
Writing on Twitter, Laurence Hill said: "Used to be roads. Now rivers of hail. Never seen anything like it."
Brighton and Hove Police posted: "Localised flooding in Hove and Portslade, some roads not passable. Please do not enter flood water on foot or in cars."
PC Andy Huggett, of Sussex Police, said: "Heavy rain after a long warm spell invariably leads to slippery roads and drivers need to immediately adjust to the new conditions.
"Along with the surface conditions, there are the added hazards of very heavy rain, flash flooding and poor visibility and it is essential that drivers make allowances."
Brighton and Hove City Council said essential services had been maintained across the city, and drains were starting to carry away the water now the rain had subsided.
"City Clean are working with East Sussex Fire and Rescue Service to clear roadside gullies.
"The refuse and recycling service is operating normally but might be a bit slower due to the weather," a spokesman said.
The council said its offices in Kings House had been closed due to flooding in the basement which was threatening some of the main electrical panels.
It said staff had relocated to other buildings, and there would be some disruption to services.