A man tried to grab or hit Paris Hilton's boyfriend as the couple headed into a Los Angeles courthouse where the socialite was to testify against a man accused of an attempted break-in at her home.
The incident outside the Superior Court building was witnessed by an Associated Press photographer and reporters who were interviewing the couple.
The man appeared to get a hand on the neck of Paris' boyfriend Cy Waits but was immediately seized by a bodyguard and taken away.
Paris has posted on her Twitter account that a "psycho intruder just punched Cy in the back of the head as we were walking into the court house".
Police officer Cleon Joseph said the man has been detained and an investigation is under way.