Images of the suspect being held to the ground by armed officers were posted to social media shortly after the alleged assault in Westminster at 3pm.
A Met spokesman said: “We are aware of commentary on social media about an incident outside Parliament.
“Shortly after 3.30pm, a man was arrested for assault after an officer was punched. The officer was not seriously injured and the incident is not being treated as terrorism.”
According to witnesses, the man could be heard shouting about how his handcuffs were on too tight.
The Met denied the officer in question was attacked to Guido Fawkes, who reported the incident was“rather a police officer recognising a person walking past Parliament who was wanted for another offence and arresting him to which the guy put up a struggle”.
The force added a 41-year-old man was arrested for assault and is being held in custody.