The M1 motorway has been closed in both directions due to reports of a "suspicious object."
Diversions are in place between between junction 14 for Milton Keynes and junction 15 for Northampton.
The suspicious object was found beneath one of the motorway bridges around 7.30am, Thames Valley Police said.
A bomb disposal unit was seen travelling along the hard shoulder towards the scene.
Members of the public have been urged to avoid the M1 as the emergency services investigate.
Thames Valley Police said road closures would likely be in place "for some time."
Three fire crews from Buckinghamshire Fire and Rescue Service were also present.
Highways England warned of "long delays" on approach to the area.
It said to expect delays of at least an hour and recommended motorists use alternative routes.
Photos shared on social media showed people walking along the hard shoulder with their suitcases.
One user reported hearing a bang. They added: "Suspicious package dealt with maybe?"
