Spokesperson Wilfred Solomons-Johannes said people started gathering from 04:00 on the N2 highway, throwing stones at passing vehicles.
He said police fired rubber bullets to disperse the crowd.
The highway was closed in both directions, between the Mew Way and Duynefontein offramps.
Motorists were being diverted onto the R300.
The highway was being cleared of debris and likely to be re-opened at 08:00.
The police were not immediately available for comment.
Western Cape Premier Helen Zille tweeted just before 07:00 that the N2 is currently being cleared and the roadway was being washed in order to reopen it as soon as possible.
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