Members of Saudi Arabia's religious police roughed up a British resident of Riyadh after they caught him paying at a women-only cash desk, local media reported on Monday.
Saudi Arabia imposes a strict interpretation of Islamic laws, notably a segregation of the sexes.
A short video posted on YouTube on Sunday shows a member of the religious police jump out of his car and attack the Briton, who was accompanied by a woman in a black abaya cloak who defended him.
"That's my wife, how dare you!" the Briton is heard shouting in the video.
Al-Hayat newspaper, quoting witnesses, said the religious police had stopped the man in a supermarket after he had paid a female cashier.
The Briton replied that he was accompanied by his wife.
Only women are allowed to pay at females-only cash desks in the kingdom. A man accompanied by female family members can stop at such desks so long as a woman deals with the cashier.
After he left the supermarket, religious police agents filmed his car, and he responded by doing the same, Al-Hayat said.
A heated exchange followed as he refused to hand over his camera to the agents, who "attacked and pushed him to the ground," according to witnesses cited by the daily.
The Briton, a convert to Islam, is a longtime resident of the kingdom and married to a Saudi woman, according to Al-Hayat, which said he was finally driven away in a British embassy car.
The religious police, named the Commission of the Promotion of Virtue and the Prevention of Vice and known unofficially as Mutawaa, have opened an investigation, Al-Hayat said.
Commission members enforce a strict segregation of the sexes and have traditionally forced women to cover from head to toe when in public.
They also patrol shops to make sure they are shuttered during prayer times.
The religious police have been accused of abusing their powers.
But since January 2013, the religious police have become more discreet after the appointment of a new chief, cleric Abdulatif bin Abdulaziz al-Sheikh, who banned the use of unmarked cars.