A 27-year-old man who stripped down to his underwear and started up a chainsaw during a meltdown at a Bunnings store has been charged.

The bizarre incident caught on video by an onlooker began after 5pm on Thursday evening inside the Bunnings store at Cannington, in southern Perth.

The man had been inside Bunnings selecting items but when he got to the counter, he tipped the goods onto the ground then allegedly removed all his clothes except a pair of underpants.

The man then walked to his parked car in his underwear and removed a baby seat, yelling "pterodactyl, pterodactyl eyes".

"When he managed to turn the chainsaw on, everyone just took a step back," witness Feraidun Hakimi told 7 News.

"It was pretty like scary… it was weird, unusual behaviour, you don't see it every day."

As onlookers gathered he jumped in his car and drove on the wrong side of the road before crashing into a fence inside a car park.

The man then allegedly removed a golf club and smashed in his car window, before pulling out a chainsaw and starting it up before alarmed onlookers.

Within minutes, police arrived on the scene and shouted at the man to get on the ground.

Channel 7 reported that after watching video of her son on television, the man's mother asked for help for her son who had once owned a business but had lost everything.

"In the state he's in he's a lethal weapon and it's scary," the unnamed woman told Chanel 7.

"More needs to be done … this stuff on the street is evil."

WA police have charged the man with drug and traffic offences.