Officers are searching Osgathorpe Park, Pitsmoor, after a man was attacked on Gayton Road at around 3.30pm.
A police cordon is in place at the entrance to the park and dogs are searching through grassland.
An eyewitness said: “I saw two cars pull up and get out with bats and knives and they have stabbed a man.
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“I saw blood pouring down his face and arm and I also saw them pull guns out.”
Anyone with any information should call police on 101 or call Crimestoppers, anonymously, on 0800 555111.