The M6 Toll motorway in Staffordshire was closed on Thursday morning following a police incident, believed to be a counter-terrorism operation.
It is believed armed police approached a stationary coach on the motorway between JT3 and JT4 at Tamworth at about 8.20am.
The coach was reported to have pulled over on to the hard shoulder of the motorway of its own volition.
The road was closed by police between JT3 and JT4 in both directions.
The intercepted coach was a Megabus travelling from Preston to London, a spokeswoman for the company said. It was heading south on the M6 toll road when it was stopped.
The spokeswoman for Megabus, which is operated by Stagecoach, said: "We are assisting police with their inquiries into an allegation made against a passenger who was travelling on board one of our services."
Bomb disposal experts were also called to the scene, while the BBC reported that a man was seen pouring liquid into a container which began smoking.
Armed police officers could be seen positioned next to the single-decker coach on the south-bound carriageway, as passengers were led off the vehicle one by one.
A number of emergency service vehicles were parked at the toll plaza, about 200 metres from the coach.
Seven police vans arrived at the scene shortly before 11am.
A spokeswoman for Staffordshire Police said: "There is currently an ongoing incident on the M6 Toll at Weeford, near Lichfield.
"Both lanes of the toll road have been closed while police deal with the incident."
The incident is not thought to be linked to Thursday morning's arrests of six people in London as part of a pre-planned counter terrorism operation.