An internal police investigation has been launched into the conduct of officers filmed arresting a teenager in Perth.

Video shows the 18-year-old being run down by an unmarked police vehicle on Wilfred Road in Thornlie, Perth, about 3pm on Sunday.

The teen was taken to Fiona Stanley Hospital, with witnesses claiming he was knocked over, hit his head on the kerb and suffered a seizure.

Locals have told 9News moments before the arrest the man had been knocking on doors asking for a cigarette and for residents to call him a taxi.

The dramatic arrest unfolded on Sunday afternoon. (9NEWS) (9news)

But at the time of the incident, the 18-year-old could be seen casually walking across the road.

The actions of the police, who have been accused of using excessive force, are now the subject of an internal investigation.

One witness who filmed the incident can be heard saying, "that's bad they hit him... that's wrong."

Local resident Dirk Jagers told 9NEWS he believes the actions by police were "excessive".

Police appear to pin down the teen in the footage. (9NEWS) (9news)

Investigations are also ontinuing into reports of suspicious activity in the area.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000