G20 protesters warned about violent gatecrashers hijacking demonstrations

Protesters are being warned that peaceful demonstrations planned for the G20 summit in Brisbane could be infiltrated by "violent and destructive elements".

Police say they know of 21 planned protests in the week or so leading up to the leaders' summit in November.

They've thanked protest groups who've registered their intention to hold events but they say there are others who won't.

"One of the greatest challenges faced by both protesters and the Queensland Police Service is that of lawful and peaceful protesters being infiltrated by violent and destructive elements," Assistant Police Commissioner Katarina Carroll said.

"Protesters need to be aware of this and remain vigilant."

She said police would not tolerate unlawful behaviour and should protesters pose a risk to personal safety or damage infrastructure, police would "take swift and decisive action".

Ms Carroll said no-one had been prohibited from attending the G20, but that could change as the event drew nearer.

About 6,000 police, including 1500 specialist officers from interstate and New Zealand, will be on duty during G20, she said.

The assistant commissioner provided the information at a community forum in Brisbane on Wednesday.