Police have told people in Scotland not to travel, as winds of up to 165mph (264km/h) affected the country, leaving more than 50,000 people without power.
As the Met Office issued its highest warning, a red alert, hundreds of schools have shut and bridge and road closures are causing disruption.
England, Wales and Northern Ireland were also being hit by wind and rain.
Winds of up to 90mph (144km/h) and possible blizzard conditions are expected in north Scotland into Friday.
The severe weather hit many other parts of the UK, including:
- Three people were rescued by helicopter after their car was swept into a swollen river in Swaledale, North Yorkshire.
- In Cumbria, heavy rain caused flooding in the Windermere area, with road closures and reports of vehicles stuck in water.
- More than 300 passengers were stranded on the Rotterdam-Hull ferry off the East Yorkshire coast. The Pride of Hull, was unable to dock, after fears that high winds could damage the P&O ship as it entered port.
- North Wales was hit hit by wind, with the strongest gust of 81mph (130km/h) recorded at Aberdaron on the tip of the Lleyn peninsula, with 70 mph (110km/h) at Mumbles Head, Swansea.
- Parts of Northern Ireland experienced disruption due to strong winds, with gales reaching 80mph along the north coast.
A gust of 91mph (146km/h) was recorded in Tiree. The ski area at Aonach Mor, just outside Fort William in the Highlands, has reported a wind gust of 130mph (209km/h ). The Met Office said winds at Cairngorm Summit had reached up to 165mph (264km/h).
Wind speeds in most populated areas were between 70-80mph.
Police in Strathclyde, Dumfries and Galloway, Central Scotland and Lothian and Borders advised against all travel until 21:00, when winds are expected to ease.
Scotland's deputy first minister, Nicola Sturgeon, said winds would affect the north east, Moray Firth, Inverness, Caithness and Orkney on Thursday night and into Friday morning.
She warned: "It is very likely that those areas will see very heavy winds, up to 90mph.
"That will also come with much colder temperatures, so there is a real risk of blizzard conditions through the later part of tonight and into the early hours of tomorrow morning as a result of that."
About 6,000 Scottish Power customers were left without electricity, with the number at about 50,000 for Scottish and Southern Energy customers.
And thousands of people in Argyll and the Western Isles have been left without electricity, Scottish Hydro said.
Dumfries, central Scotland and the Clyde coast were badly hit with power cuts, after trees and other debris blew onto and, in some cases, brought down overhead power lines.
Scottish Power said it had more than 600 engineers battling to restore power, with extra workers being drafted in from Merseyside and Wales.
But it warned fallen trees were blocking access, and high wind speeds prevented engineers from climbing poles.
Localised faults were also reported in Sanquhar, Dumfries, Dollar, Falkirk, Helensburgh and Stranraer.
Scottish Power said later that more than 18,000 customers were reconnected, from 17:00.
Two hospitals - Belford in Fort William and the Victoria in Rothesay - had to switch to back-up generators after they lost power, and patients were evacuated from Kirkcaldy's day surgery after the roof blew off.
Central Scotland Police Assistant Chief Constable Allan Moffat, speaking on behalf of the Association of Chief Police Officers in Scotland, said: "The advice for motorists across the central belt of Scotland is to avoid travel, as the severe weather moves across the country from west to east, starting around noon on the west side of the country.
"It is expected that the impact of the weather will affect the east side of the country from 2pm onwards.
"This advice to avoid travel is not given lightly but is based on the clearest information yet from weather forecasters that there will be high winds with gusts of up to 90 miles per hour."
The Forth Road Bridge was closed to all traffic, after a gust of 84mph was recorded, but has now reopened to cars only.
The Scottish government's resilience committee has been meeting to assess the changing situation.
Police are warning of the potential for serious travel disruption, with gusts of more than 80mph (128km/h) expected in places.
Flooding in a number of areas has also caused problems, including road closures.
Train operator ScotRail said it had been forced to cut services, with a temporary timetable coming into force, as Network Rail imposed a 50mph (80km/h) speed restriction.
Commuters were also advised to consider leaving work early because of the possibility of afternoon rush-hour disruption.
The government said schools across west, central and southern Scotland should either not open or close early.
Decisions on closures are a matter for local authorities, but the government issued advice on Wednesday night that schools in the west should consider not opening, and schools in the east should close by Thursday lunchtime.
The Met Office said there was significant risk of structural damage.
Parents have been advised to check their local council websites for information on schools for Friday.
Network Rail managing director for Scotland, David Simpson, said: "We will have over 350 engineers out on the network, working to keep the railway running.
"However, the extreme nature of the conditions, and the impact they can have on our infrastructure, means that a speed restriction is necessary in the interests of safety."
Police in the Highlands have warned drivers to prepare thoroughly for any journeys they have to make, while Tayside Police said fresh falls of snow and blizzard conditions were expected in places.
All double-decker buses have been withdrawn by First Glasgow. A much-reduced service will run using single-decker vehicles.
Another casualty of the weather is the Christmas display in Glasgow's George Square. Glasgow Life has said the festive attractions will be closed all day on Thursday as a precaution.
Elsewhere, heavy rain has led to flooding in the Windermere area of Cumbria.
Local police said they had a number of reports of vehicles stuck in water around Windermere and Troutbeck.
Sections of the A591 at Ambleside and Windermere were also closed because of the flooding, along with a section of the A592 at Troutbeck Bridge.
And there were strong wind warnings for the M6 in Cumbria and the A66 in County Durham was closed to high-sided vehicles.
Parts of Northern Ireland have also experienced disruption due to strong winds, with gales forecast to reach 80mph along the north coast.