Forty-one people were left injured after a coach overturned on the M25 near Swanley in Kent.
Three people with more serious injuries were taken to hospital, while 38 were treated for minor injuries.
Seven children were among those hurt, South East Coast Ambulance Service said.
In a tweet following the crash, Kent Police said it was "proud to see all emergency services working so effectively together" and confirmed that, "to make things interesting", a baby boy was born at the scene.
The Princess Royal University Hospital in Orpington, Kent, is on major incident alert, a spokesman said.
Lanes one and two at Junction Three anti-clockwise have been closed, as has the exit slip road, causing major delays.
A 51-minute delay is expected with traffic queues stretching back five miles, Highways Agency said.Police, ambulance and fire services were called to the scene at the M25 at Swanley just before 4pm.
A statement from Kent Police said: “Kent Police was called at approximately 4pm on Monday August 13 to reports of a collision involving a coach on the anti-clockwise carriageway of the M25 near to junction three.
“Officers are currently at the scene with South East Coast Ambulance Service and Kent Fire and Rescue Service.
“The Junction Three slip road on the anti-clockwise carriageway and lanes one and two are currently closed.
“There are delays to traffic and motorists are advised to seek alternative routes where possible.”
A spokesman for Kent Fire and Rescue Service earlier said firefighters were using cutting gear to free two people trapped.
Three fire engines and “several vehicles from the heavy rescue team” were sent out “as a precaution”, he added.
Police are urging motorists to plan ahead.