The earliest widespread snowfall for 17 years has gripped many parts of the UK.
Up to 15cm (6in) settled in northern Scotland and north-east England, with icy roads making driving hazardous.
The Met Office issued heavy snow warnings for large parts of Scotland and parts of north-east and eastern England, Wales, Yorkshire and Humber, and the East Midlands.
Forecasters said the cold spell could last a fortnight, with snow reaching England's southern counties next week.
Temperatures are expected to drop to -5C in some areas over the next few days, far lower than those normally experienced in November.
It is unlikely to rise much above 2C-5C by day, remaining lower in the more exposed, rural areas.
Breakdown surge
Northern and eastern parts of the UK are expected to bear the brunt of the wintry conditions.
Chris Fawkes, from the BBC Weather Centre, said 5-13cm (2-5in) of snow were expected in low-lying areas later, but places like the North York Moors, which were particularly exposed to the north-east wind, could see falls of up to 20cm.
"Winter has arrived early this year," he said. "This cold snap is probably going to last the best part of a fortnight."
He said many central parts of the UK would be dry and sunny by day, but cold and icy by night, and snow was likely to hit southern counties of England next week.
Some 15cm (6in) of snow fell overnight in Aberdeenshire, 12cm (5in) in the Scottish Borders and 10cm (4in) in Durham. Problems with snow and ice were also reported in Northern Ireland, Wales and south-west England.
Motorists were urged to exercise caution on the roads as temperatures plunged.
The AA reported a surge in breakdowns and said people should be particularly wary of treacherous black ice on Thursday evening.
The rescue service said there had been 9,100 call-outs by 1500 GMT, and it expected that figure to rise to 14,000 by the end of the day.
Spokesman Gavin Hill-Smith said call-outs were up around 50% compared to average November levels, with some 1,000 an hour. Parts of Tyneside and Yorkshire were even higher, he added.
"Black ice is far more treacherous than snow, people have to keep speeds right down this evening, particularly if travelling in more rural areas," he said.
Rock salt sales
He advised people to stick to main roads wherever possible, and to override their sat-navs when necessary.
Some councils have already said they would not be gritting as many roads this year to save money.
But Councillor Richard Kemp, from the Local Government Association, said: "We've got more [grit] in stock at the moment than we've ever had before. We've got more on order. It is now a question of when we can get those orders through."
Across Scotland, the worst affected areas are Grampian, East Lothian and the Borders, with the snow forecast to spread to the rest of the country by the weekend. More than 120 schools were affected in Aberdeenshire.
Grampian Police say road conditions throughout the region are treacherous and are urging motorists not to travel unless it is absolutely necessary.
Police said all roads in the Grampian region had snow and ice, and closed roads included the B974, the A939 at the Lecht ski centre, the A93 at Cairnwell, and the B976 Crathie to Gairnside.
Aberdeenshire Council said gritters, snowploughs and diggers have been out clearing the roads.
The BBC's James Cook, in Aberdeenshire, said the Scottish government had been stockpiling grit to avoid last year's problems when supplies ran out.
North Yorkshire woke up to a blanket of snow on Thursday morning, and 10 schools in the county have closed.
Traffic delays
A spokesman for North Yorkshire County Council said all major roads were open, including the A169 Pickering to Whitby road, which had been badly affected.
The A170 at Sutton Bank was particularly affected by snow, as was the B1249 at Staxton Bank near Scarborough, and the A165 Reighton bypass was partially blocked.
In Cornwall, lorries and coaches have been getting stuck on some roads, with accidents and delays reported on the A30.
Highways staff have been pre-treating 25 roads across the county with salt ahead of more snow and ice forecast for the next few days.
The highest parts of County Durham and Northumberland have seen 15cm (6in) of snow, and there have been reports of accidents and smaller roads not being gritted.
A car overturned on the A1 southbound in Gateshead, and in County Durham one lane of the A1 northbound was blocked between Carrville and Chester-le-Street because of a jack-knifed lorry.
Nearly 40 schools across north-east England have closed as well as a handful in rural parts of north Yorkshire.
The wintry weather forced Durham Tees Valley airport to close for part of the morning, but by 1300 GMT it had reopened.
Rail services have been unaffected, the Association of Train Operating Companies said.
The prospect of continued snow has sparked a surge in rock salt sales of 1,600%, according to DIY chain B&Q.
Wellington boots were also flying off the shelves, it said.
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