The New Forest was covered in snow and more was expected later
Heavy snow has fallen overnight across much of Hampshire, Berkshire and Dorset severely disrupting travel.
Forecasters said it was the most severe snow in the UK for two decades.
Hundreds of schools have been shut and train and bus services suspended. Motorists have been advised to "only make necessary journeys".
The snow also caused the runway at Southampton Airport to shut for several hours. It later reopened but many flights were delayed as a result.
Up to 6in (15cm) of snow fell overnight and more snow is expected throughout the day and over the next few days.
It led to the Met Office issuing an extreme weather warning for the south east of England.
More than 500 schools in Hampshire, 255 in Berkshire and about 70 in Dorset were shut.
Marwell Zoo, near Winchester, was also closed "due to the current adverse weather conditions".
First Great Western (FGW) services between London Paddington and Reading, Berkshire, and the west country were disrupted due to "adverse weather conditions".
FGW was advising people not to travel unless necessary.
South West Trains was running a reduced service from London to Hampshire and Dorset.
The A338 Bournemouth Spur Road was shut in both directions but later reopened.
Only one lane of the northbound M3 at Winchester and the M275 and Eastern Road in Portsmouth were open.
The A27 between Havant and Chichester and the A331 Blackwater Valley and A325 Farnborough to Aldershot roads were seriously affected, as were many rural roads in Dorset.
Hampshire police urged motorists to take "extreme care and give plenty of extra time for their journeys".
Some roads have been described as treacherous
They said the north of the county around Basingstoke, Farnborough, Aldershot, Yateley, was the worst affected area, with some roads left impassable.
The Highways Agency said it was continuing to grit all trunk roads and main roads.
But it warned motorists many roads, in particular B roads and C roads which gritters had not been able to reach, were treacherous.
A spokeswoman for Hampshire police said: "Motorists are advised to only make necessary journeys at this time, do not travel unless you have to.
"If you do have to drive, take plenty of extra time, avoid heavy braking, and ensure you have a blanket, warm clothes and it is highly advisable to pack a hot drink."
Heathrow Airport has also had to shut one of its two runways and said it would take about two hours to reopen. There were "significant" delays and cancellations.
Tom Defty, head of forecasting operations at MetService, said some areas of high ground could see as much as 8in (20cm) of snow.
Icy winds are also making temperatures feel "sub zero" as the coldest winter in 13 years continues.
Night-time temperatures are set to drop to -2C (28F) as a blast of cold air sweeps in from the North Sea.
Slightly milder air will move in during Tuesday, with snow mainly confined to hills and in other areas precipitation is expected to turn to rain.