A man suffered face and head injuries in what police have called a case of mistaken identity by vigilantes.
The 38-year-old man was assaulted on Woodhorn Lane, Ashington, at about 18:30 BST on Saturday.
Northumbria Police said it is believed he was attacked after being mistaken for a man accused of sharing indecent posts on social media.
Officers said that that man had been detained under the Mental Health Act and no longer lived in Ashington.
The attacked man did not need hospital treatment but has been "left shaken", a force spokesman said.
Ch Insp Ron Charlton said police were looking for the attackers in order to prosecute them.