North Yorkshire Police said they were called to a “suspected violent incident” at the Beer and Burger on Parliament Street just after 5.30pm.
A police spokesman said: “A 72-year-old man suffered what appears to be a serious head injury. He was taken by ambulance to hospital where he is receiving emergency treatment.
“A 24-year-old man has been arrested in connection with the incident and is being questioned in custody.
“Witnesses are asked to call North Yorkshire Police on 101. Or call Crimestoppers, anonymously, on 0800 555111.
“Please quote reference NYP-11062015-0379 when providing information about this incident.”