NSW Police Commissioner Andrew Scipione says he was outraged to see children holding signs that called for the beheading of anti-Islamists at a Sydney protest.

Mr Scipione said it was the actions of "extremist offenders" who turned a peaceful protest violent on Saturday, leaving six police and 17 others injured.

Andrew Scipione ... says social media played a role in the protest. Credit:James Alcock

"It was an outrage," he told reporters in Sydney today.