The incident reportedly occurred near the Capitol Centre in London Road at approximately 4pm.
Video footage from the scene shows a large emergency presence in the area and blue tarp covering the police van.
The fire service has confirmed crews rescued one casualty from one of the vehicles involved.
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"We can confirm a British Transport Police vehicle was involved in a collision on London Road in Preston," a spokesman said.
"The incident remains ongoing with other emergency services at the scene."
The road has been closed for hours following the collision, with delays of approximately 25 minutes reported.
Lancashire Police and North West Ambulance Service have been approached for comment.
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