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Aerial footage shows passengers surrounded by police on the otherwise deserted motorway
Armed police have swooped on a coach on the M6 Toll motorway in the West Midlands.
A police source told BBC Radio WM a passenger was seen pouring a liquid into a box, which then started smoking.
The 48 passengers on the Megabus Preston to London service were led off the coach and forced to sit apart in a cordon on the opposite carriageway.
The motorway was closed in both directions between junctions T3 and T4 at Weeford Park Plaza, near Lichfield.
Police said the coach stopped on the hard shoulder "of its own accord" at 0820 BST.
Sniffer dogs A Megabus spokeswoman said the coach left Preston at 05:10 and was due in London at 10:55.
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Nick Jones EyewitnessIt all just took off, ambulances left right and centre and armed police in black caps"
She said: "We are assisting police with their enquiries into an allegation made against a passenger who was travelling on board one of our services."
Armed police officers could be seen next to the single-decker coach on the southbound carriageway, as passengers were led off one by one.
They were taken to the northbound carriageway where they were made to sit on the road, apart from one another, while surrounded by officers.
Meanwhile, sniffer dogs were brought in to aid the search of the coach.
Air con warning Nick Jones, a sales manager from Cambridge, was caught up in the incident while driving to the Lake District.
He said he was about four vehicles behind the coach when police stopped traffic.
"I've never seen so many ambulances turning up, also armed police, helicopter and dozens of police cars.
Passengers were taken off the coach and made to sit on the carriageway "We were told to stay in our cars, keep windows up and not put air conditioning on.
"Police were looking in hedgerows and verges on the northbound carriageway.
"It all just took off, ambulances left right and centre and armed police in black caps."
Another eye-witness Barry Jarvis told the BBC that cars were cleared from toll booths.
He said: "It was very puzzling as to what was going on at first as there weren't that many police there.
"It was only after 20 minutes or so when all these unmarked cars coming through that we thought something major was happening.
"As we were diverted, we could see armed police at the toll booths, and the fire brigade."
The fire service, police from the West Midlands and Staffordshire forces and ambulance crews are at the scene.
The Highways Agency said the closure was likely to remain in place until early afternoon.
Diversions are in place and trapped traffic was being turned around, police said.
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