A 76-year-old man needed surgery to have 14 screws put into his broken ankle and treatment for a broken nose after he was attacked by at least four youths.
He was walking alone through the Garden of Remembrance in the Causeway, Horsham, when he was set upon.
His bag containing a mobile phone and glasses was stolen.
At about the same time, an elderly woman in the park had her walking stick kicked away and her shopping trolley pushed into bushes.
Furniture was overturned in St Mary's Church and police think the incidents may be related.
Detective Constable Tom Cashmore, of Horsham CID, urged witnesses to come forward and said: "This was a very nasty attack on an elderly man."
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