A driver was killed when a falling tree hit his car as strong winds, rain and snow swept into the UK.
The warning, which is the highest the Met Office issues, means the storm is likely to be severe with the potential for damage to buildings and homes, with roofs blown off and power lines brought down.
Winds of up to 75mph lashed Northern Ireland, leading to a number of trees falling, causing disruption on the roads. Ferry sailings from Belfast and Larne ports were cancelled.
Forecasters said Storm Arwen would bring high winds and disruption for much of the UK.
The red weather warning for wind, lasting from 3pm on Friday until 2am on Saturday, was added to amber and yellow wind warnings for disruptive weather.
The alert also warns people of the potential for roads, bridges and railway lines to be closed, with delays and cancellations to bus, train, ferry services and flights.
Rod Dennis, of RAC Breakdown, warned of the chance of major disruption and urged motorists to “avoid driving if at all possible”.
“Red warnings from the Met Office are relatively rare and are the strongest possible signal to drivers not to set out in the first place unless absolutely necessary,” he said.
Coastal areas in the east of Scotland and the northeast of England were predicted to experience the highest winds, with gusts in excess of 80mph expected.
A senior police officer warned drivers in the red weather warning areas they “should not travel under any circumstances”.
In Scotland, a section of the A1 was closed in East Lothian because winds of up to 84mph were forecast for Friday evening. Motorists were warned to expect delays.
Alex Deakin, of the Met Office, said there was potential for “very dangerous conditions, unusually strong winds and very large, powerful waves”.
It could cause structural damage, power cuts and travel disruption, he warned.
Another amber warning was in place for north and west Wales and southwest England.
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