POLICE are expecting about 20 protests today, with the biggest beginning at 11am.

BrisCAN G20 People's March began at 11am with speeches, and protesters will start their walk from the Roma St forum to Musgrave Park in South Brisbane at midday.

Police have described today as their "biggest concern" in terms of protests.

"We will also see probably the highest volume of protest activity we have seen to date," Queensland Police Service Deputy Commissioner Ross Barnett said.

"The reality is this is the most significant peace time security event that Australia has seen."

Police logistical units are travelling around the city keeping patrolling officers hydrated.

Queensland Police Service Assistant Commissioner Katarina Carroll said many officers would walk up to 20km a day in their 12 hour shifts.

There have been no G20-related arrests or bans overnight.