A 40-year-old man was arrested yesterday after allegedly assaulting the owner of a Yau Ma Tei restaurant after the owner was involved in a quarrel with an armed off-duty detective. The man was taken into custody after the detective, a Shamshuipo district crime squad constable, also 40, was arrested for allegedly pulling out his gun during an argument with the owner on Tuesday. Last night, both men were being held for questioning in Kowloon City police station. No charges have been filed yet. Officers from Kowloon West regional crime unit were still looking for more than 20 people who were involved in the assault. Apparently the owner, 52, first promised to give the officer and his friends free beers as a promotional offer at the Nathan Road restaurant early on Tuesday. But he changed his mind when other customers overheard the offer and also demanded free beers. A dispute erupted when the officer and his friends called for the bill and discovered they were being charged for the beers. A source said that during the argument one of the officer's friends claimed to be a triad member. The group finally paid the bill and left the restaurant. When the owner was leaving the restaurant at 4am he was assaulted by more than 20 unarmed men, who fled before police arrived. He was treated at Queen Elizabeth Hospital for head and arm injuries. Police Commissioner Tsang Yam-pui yesterday refused to comment on the case. But he did say the force would not tolerate any officer involved in illegal activities. Detectives are allowed to carry guns while they are off duty. If they decide to leave the gun at home, they must ensure it is kept in a safe place. Under police general orders, weapons should be left unloaded when not carried.