The man, who was later identified as Shabir Ahmad Khan, yesterday slapped the 37-year-old Imam at the National Mosque in Kuala Lumpur.
"The man was an Indian national from Kashmir and had walked to the front and tried to shove the Imam Mohd Zuhairee," the Royal Malaysian police said.
"After that, the suspect went straight to the complainant (Imam) and pushed him from his place and tried to take the microphone that he was using.
"The suspect later slapped the complainant once and immediately took the Imam's spot to lead the prayer," police said.
It was not clear what triggered the man to attack the Imam. According to police, preliminary investigation found that the suspect was of unsound mind as he was seen speaking to himself.
The incident momentarily halted the Friday prayers which were resumed shortly after the suspect was escorted away, police said.