Thousands of homes are without power and commuters are facing severe travel disruption after the worst storm in years lashed the UK.
Winds of almost 100mph have left houses across large parts of the South and East without electricity.
More than 40 trees have been cleared off railway lines, and at least 100 trees have been discovered on lines across the South East so far, Network Rail said.
In central London, Whitehall was closed both ways between Parliament Square and Horse Guards Avenue after a crane collapsed on to the roof of the Cabinet Office.
London Mayor Boris Johnson will chair an emergency resilience meeting involving all emergency services and relevant agencies later this morning.
Tracy Elsey, communications manager for UK Power Networks, told the BBC Radio 4 Today programme: "We have got close to 140,000 people without power at the moment.
"We have still got about 40,000 people off in the South East, which is our region which runs from Brighton up to Kent.
"In our East of England region, which goes from Essex up to the North Norfolk coast, we have seen a huge rise in reported power cuts. We have got 100,000 properties off power in the East."
Scottish and Southern Energy (SSE) said more than 38,000 customers were left without power in Cornwall, Devon and Somerset, while Western Power reported more than 3,800 power cuts in the same areas, Downing Street said.
Trains across the country have been disrupted, with many operators not expecting to run services until later in the day.
Police said at least 125 trees were down across roads in Sussex by 6.30am, and Kent Police said at least 70 trees had been blown down across the county.
Transport for London (TfL) said there was disruption to six Underground lines due to debris from the storm on the tracks.
The Bakerloo, Central, Jubilee, Metropolitan, Northern and Piccadilly lines were all partially closed while workers removed fallen trees and other obstructions, a TfL spokesman said.
The Environment Agency has 137 flood alerts in place across England and Wales, warning people to be prepared, and 13 flood warnings, with 12 in the South West.
Winds of up to 80mph have been reported, while a gust of 99mph was recorded by the Met Office at the Isle of Wight at 5am.
Major roads around the country have been closed, including both Severn crossings and the A249 Sheppey Crossing in Kent.
Mr Johnson said: "Clearly this has been a difficult night for many Londoners, and continues to be an incredibly trying morning," he said.
"Transport for London, the boroughs and the emergency services are working flat out in an effort to keep London moving and minimise disruption as far as is possible.
"I want to thank all the agencies for their professional response in incredibly testing conditions, and I'd urge Londoners to check before travelling by going to tfl.gov.uk for the latest information."
London's Royal Parks have also been closed because of the weather conditions.
A spokeswoman said: "The closures are a necessary precaution to ensure the safety of all park users including vehicle users and cyclists.
"The parks will reopen as soon as The Royal Parks management and the police are satisfied it is safe to do so. We are aware of some damage to trees."
About 40 houses were evacuated after a fallen tree caused a gas leak in Reading, Berkshire.
Thames Valley Police said the leak was near Calcot Golf Club.
Police and firefighters from the Berkshire Fire and Rescue Service have enforced a 612-yard (560m) cordon for public safety.
Mike Cooper, a director at Richfords Fire and Flood Restoration Specialists, said one home in Camborne, Cornwall, had to be evacuated overnight when a tree fell on to the property.
But he said people had done their best to guard against being caught out by the weather.
He said: "It looks as though the incident in Camborne happened without causing any injury, so obviously everyone is thankful that is the case.
"We've had a handful of incidents reported to us overnight and, for a county which has coastal borders on three sides, some localised flooding has to be expected.
"But I think people have taken heed of the warnings and prepared as best they can, which is good.
"Whenever this is serious risk to life - as there was with this storm - then it is very hard to argue that its potential has been over-emphasised. Thankfully it looks like most people have been sensible, kept safe, and reduced the risk to themselves and their property."
The collapsed crane in Whitehall caused Deputy Prime Minister Nick Clegg to cancel a scheduled press conference.
"Today's press conference moved to a day when there isn't a crane on the roof and journalists travelling on the train are able to join us," he wrote on Twitter.
Robin Gisby, Network Rail's (NR) managing director of network operations, said: "NR is dealing with this severe weather in exactly the way that we and the train operators planned over the weekend.
"We have had several hundred staff on duty through the night and into the morning to monitor conditions and react to any damage or disruption caused by hurricane-force winds. Safety remains our top priority."
He went on: "While conditions were as forecast during the early part of the morning, the damage caused by the storm has been more severe than expected as it has tracked eastwards to the north of London and across to East Anglia.
"As a result, the West Coast, East Coast and Midland main lines are all currently blocked at their southern ends as a result of fallen trees and damage to power lines and all services are currently suspended on the Anglia route, where the storm is currently."
He added that he was hopeful that at around 9am services would begin to resume south of London once obstructions had been cleared from lines.
Mr Gisby went on: " At the latest count we have had more than 100 trees down across the southern half of the country and we expect to find more as we complete our safety checks this morning."
A Cabinet Office spokeswoman said its staff have all been evacuated while the extent of the damage caused by the crane is assessed and that none were injured in the incident, which happened at around 6.50am.
"As is the case with many other buildings in storm-affected areas, and despite the necessary preparations, there has been some damage to the Cabinet Office building," she said.
"A crane at the back of the Cabinet Office was dislodged by the storm, and is currently resting on the roof. We are assessing the damage, and the building has been evacuated while we do so.
"Many staff can log on and work flexibly from the Treasury building or from other locations and we are exploring other temporary options.
"We are working hard to ensure the safety of our staff and minimum disruption to operations."
An oak tree damaged three cars and blocked a street in central London.
The tree, which smashed two windscreens and crushed the bonnet of a Peugeot, was discovered in St George's Square, Pimlico, early this morning.
It is expected to be removed by Westminster Council by this afternoon.
Annabel Colgrain, the owner of a 2006 Volkswagen which was trapped by the tree, said: "We don't know if it can be driven but I had a pretty lucky escape. I've got a few branches down over the roof.
"I was in Kent for the storm of '87 and then we were completely cut off. We did not have television or phone lines until Christmas, for at least six weeks. This is nothing by comparing to that. I'm well-versed in British storms now."