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Part of the M20 was shut in both directions after emergency services were called to welfare concerns for a man.
There are six-mile delays after traffic was held between Junction 8 for Leeds Castle and Junction 9 for Ashford.
A diversion route was put in place and motorists have been urged to allow extra journey time.
The road has since reopened but there are major delays after the road was closed.
A post shared on social media by National Highways said: "The M20 has now reopened in both directions between Junction 9 (Ashford) and Junction 8(Maidstone) following an earlier police-led incident.
"There are six miles of congestion following this incident on the west carriageway and three miles on the east carriageway."
A police spokesman says the force was called shortly after 3.30pm to "concerns for the welfare of a pedestrian" on the Londonbound carriageway of the M20 between junctions 9 and 8.
"Officers and South East Coast Ambulance Service attended the scene and a man was passed into the care of paramedics," they said.
"Both carriageways were closed while emergency services dealt with the incident and have now reopened."
Nick Wood was stuck on the M20 between Junctions 8 and 9 for almost an hour.
"I'm stuck on a bend so can't see anything ahead," he said.
"It would have been about 4.20pm when I came to a stop and I think, I’m about half way between Leeds Castle and Ashford."
Road users travelling eastbound were told to follow the hollow circle diversion symbol on road signs.
Motorists should exit the M20 at Junction 8 and take the third exit at the roundabout and at the next roundabout take the first exit on to the A20.
Stay on the A20 until the junction with Fougeres Way where it will will turn northwards towards Junction 9 of the M20 and rejoin the M20 eastbound at Junction 9.
Meanwhile, road users travelling westbound were urged to follow the solid square diversion symbol on road signs.
Exit the M20 at Junction 9 onto Fougeres Way and at the next roundabout take the fourth exit onto the A20 heading west.
Follow the A20 back to the M20 Junction 8 and at the roundabout take the first exit onto the M20 westbound.
In a tweet, KCC Highways said: "Traffic is stopped in both directions near Junction 9 due to a police-led incident. Expect delays in the area."
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