The "Spoofing the Traffic Camera" video, which has notched up more than 200,000 YouTube hits, shows skater Decio Lourenco, hands folded behind his back to decrease wind resistance, hurtling down the mountain road in fading light.
Motorists on the opposite side have their headlights on.
Toward the bottom of the run, he is seen throwing up his hands in jubilation after a GPS system strapped to his foot showed he had clocked up 68 mph in a 37 mph zone, local media said.
"We have handed over the matter to traffic officials and want him arrested," JP Smith, the Cape Town official in charge of safety and security, told Reuters.
"The speed limit is irrelevant. He is seen careering down the road in a reckless and dangerous manner."
The road in question, Kloof Nek, is one of Cape Town's steepest and has been used to host extreme skateboarding events, although only when closed to traffic. The city does not allow skateboarding on public roads.
The threat of arrest for one of their most celebrated comrades has the close-knit skateboarding community up in arms, with posts on the National Skate Collective Facebook page calling for "mass action."
"The city is trying to shut us down," read the page, which carries the popular skater tag-line "Skate or Die."