Emergency services are on the scene dealing with a multiple‑vehicle crash near the junction by Moyles Court on the A338, Ringwood–Salisbury Road.
The southbound carriageway is completely blocked.
Four people have been taken to the hospital, some with serious injuries, police say.
One person was trapped inside a vehicle before the firefighters were able to get them out.
Members of the public who stepped up to assist have also been thanked by emergency services, including a nurse practitioner, a retired firefighter and a social worker.
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Drivers are advised to expect delays.
A police spokesperson said: “We were called at 2.08pm today, July 6, to reports of a collision between three cars on the A338 in Ringwood.
“Four people have been taken to the hospital for treatment, some with serious injuries.
“A road closure has been in place, but this is expected to be lifted soon.”
A spokesperson for the fire service said: “A crew from Ringwood alongside one from Dorset and Wiltshire FRS, attended a crash involving three vehicles on the Salisbury Road this afternoon. The initial call came in just after 2pm.
“Firefighters released one person who was trapped inside one of the vehicles. Once released, the person was passed into the care of the ambulance service alongside two other casualties.
“We would like to thank members of the public who stopped to help at this incident, including a nurse practitioner, a retired firefighter and a social worker.
“The road has been closed while the incident is being dealt with and will remain closed for a while to allow for vehicle recovery.”