Forensics teams were at The Level at the scene of an alleged stabbing.
Police were called to the park in Brighton just before 4.15pm on Friday.
A section of the park was cordoned off and forensics teams could be seen investigating a pile of clothes.
Police remained in the area all evening and said to expect a heightened police presence while they continue their enquiries.
A man was stabbed and taken to hospital with “serious injuries”.
Police believe it to be an isolated incident and there is “no danger to the wider community”.
A spokesman added: “Anyone with information which could help officers with their enquiries is asked to report it to police online or call 101 quoting Operation Danegate.
“Alternatively information can reported anonymously to the independent charity Crimestoppers on 0800 555111.”
A 30-year-old man was arrested on suspicion of causing grievous bodily harm with intent and has been released on conditional bail until November 18.
Yesterday afternoon, a man was “struck with a bottle” in Montague Street in Worthing town centre.
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