A footbridge has collapsed on the M20 motorway in Kent after it was hit by a lorry carrying a digger, on one of the busiest travel days of the year.
The motorway is closed between junctions 1 and 4 after the bridge fell on to the London-bound carriageway.
A motorcyclist was hurt but his injuries are not life-threatening.
Travel disruption is said to be severe, with the motorway - the main route to the Channel Tunnel - expected to remain closed until at least midday on Sunday.
Part of the bridge also fell on to a second lorry, during the incident which happened shortly after 12:05 BST. The driver was treated for shock at the scene.
South East Coast Ambulance Service tweeted that a motorcyclist in his 70s had been taken by ambulance to Tunbridge Wells Hospital with suspected broken ribs after the bridge collapsed.
Officers from Kent Police have now left the scene but the force said it "continues to support Highways England in ensuring the motorway is reopened as quickly as possible".
The force said no-one had been arrested in connection with the incident.
Local roads including the A20 are congested and motorists are advised to avoid the area and find alternative routes if possible.
Gary Coleman from Highways England said: "We are facing a real challenge to lift two HGVs and a motorbike clear of the scene and deal with all of the rubble from the collapsed bridge strewn across both carriageways.
"There is also the issue of the remaining part of the footbridge, which is still in place over the coast-bound side of the motorway. We have crews on scene ready to take action as soon as the police have completed their investigation. We also have cranes en route and lighting so work can continue through the night.
"We are doing everything we can to safely reopen the motorway."
The crash happened near the junction with the M26, which links the M25 London orbital with the M20.
Kent Police said it was investigating the circumstances of the crash and appealed for witnesses to get in touch by calling the appeal line on 01622 798538 and quoting the reference 27-0690.
The crash had knock-on effects across the motorway network after the eastbound M26 was closed to stop further congestion in the area, leading to queues on the M25, which travel reports said was "effectively down to two lanes".
About three hours after the collision, all traffic trapped on the southbound M20 had been released.
Cross-Channel traffic is being diverted on to the A2 and M2 from the M25 and the Port of Dover, Highways England said.
'Slammed on brakes'
Sonny Gladdish was on the M20 about four or five cars behind the crash when it happened.
He said a lorry carrying a digger was driving on the hard shoulder and hit the bridge with a loud bang.
He said the bridge collapsed on one lorry and he believed the motorcyclist had thrown himself off his bike to avoid the impact.
Another motorist, Fade Ogunro, said: "I was driving down the M20, looked up and slammed on the brakes.
"Across the road a footbridge had literally just collapsed."
She said everyone "went into panic mode" and that people were unsure about whether to approach the vehicles involved or to drive under the bridge on the other side for safety reasons.
"We were all just like in a state of shock. It was something like out of a movie."
Witness Andy Sunnucks, 24, a graphic designer who was travelling from Maidstone to Sevenoaks, said: "It looked like a lorry had jack-knifed, and I could see half the bridge was missing. We went to have a look and the back end of the lorry was in pieces."
Alex Magaisa, his wife and two young sons were driving to Gatwick Airport and were just passing the lorry when the bridge collapsed.
He said: "My instinct was just to drive through. My wife saw the bridge falling and there was a big noise. I had to manoeuvre through the debris.
"It was a big shock. It's only just starting to sink in now what might have happened. We could have been crushed."
Another witness, Sean Fisher, said: "I spoke to [the woman in] the first car behind the lorry. She seemed to be all right. Her daughter was quite brave and went under the bridge and was the first responder to the fallen motorcyclist."
Some drivers also tried to stop traffic on the other side of the road, Mr Fisher said, as they feared the bridge might collapse on the coast-bound carriageway as well.
People took to social media to share pictures of the collapsed structure.
Helen Ayres posted: "Now that's what I call timing. Just as well the M20 is usually free-flowing on a Bank Holid... Oh. 😣"
Jimmy tweeted: "Couple of guys playing football on the m20; not something you see everyday #m20 #motorway #traffic ..."
Some drivers objected to the handling of the aftermath of the crash, with William Gaunt tweeting: "@kent_police why so long to close M20? 1000s joined 45 mins after bridge strike now trapped."