Hundreds of Aboriginal activists have marked what the say is invasion day by blocking the annual Australia Day cultural parade through Adelaide.

The last sections of the parade, which featured about 3000 people from more than 100 cultural groups, were abandoned as protesters fanned out across the route in what they said was a show of defiance.

Earlier, they forced police and organisers to divert the parade onto footpaths as it made its way down King William Street in the heart of the city.

To chants of "always was, always will be Aboriginal land" the group was watched by a large crowd of revellers who had come to see the usual Australia Day entertainment, including a concert and fireworks.

Instead they were witness to one of the largest protests to take to the city streets for some time, with the activists watched over by a strong police and security presence.

Police were still to comment but there appeared to be no violent incidents or arrests.

One of the organisers, Latoya Rule, used the occasion to shave her head to highlight the suffering of Aboriginal people.

After blocking the parade, the activists moved on to SA's parliament house, staging another brief protest which blocked the intersection of King William Street and North Terrace for several minutes, forcing police into some hasty traffic control.