Officers are trying to establish how a man suffered head injuries and are appealing for witnesses who may have seen what happened.
He was taken to the Queen's Medical Centre after being found in Cromwell Street, Mansfield, at 5.30pm on Wednesday (May 29).
His condition is serious but stable and officers have not yet been able to speak to him.
Enquiries are ongoing but anyone who saw what happened to the man is urged to call Nottinghamshire Police on 101, quoting incident number 678 of 29 May 2019, or call Crimestoppers anonymously on 0800 555 111.
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