The road is closed in both directions between the Bramham crossroads and Tadcaster.
North Yorkshire Police confirmed the closure at 1.30pm on Saturday, July 3.
The force said: "The A64 near Tadcaster is closed in both directions between the A162 and the Bramham crossroads, while we deal with an incident.
"Diversions are in place - thank you for your patience, and please avoid the area."
Highways England said: "A64 westbound between Tadcaster and the A1M.
"The carriageway is currently closed due to a North Yorkshire Police led incident.
"We are heading to assist but likely to cause delays in Tadcaster with traffic diverting locally at this time.
It later added: "The A64 is closed in both directions between the A1M and Tadcaster.
"Very heavy traffic in the area."
The AA reports severe traffic in the area with delays increasing, particularly in Tadcaster village.
It also reports queuing traffic on the A1 at Aberford
Average speed in some areas is reported to be five miles per hour.
At 3.10pm, North Yorkshire Police confirmed that the A64 eastbound has now been released and is open.
Westbound remains closed from the A162 Tadcaster junction towards the A1m.
The police confirmed that there are just over 2.5 miles of slow moving traffic on approach to the incident and delays of 30 mins above normal journey time for this time of day.