More than 6,000 police officers from as far afield as New Zealand have locked down the centre of Australia's third-biggest city, while F/A18 Super Hornets will enforce a strict no-fly zone around the Brisbane Convention and Exhibition Centre.
Police have been given boosted powers, including the ability to strip-search anyone deemed suspicious near the venue, to protect attendees including Obama and the leaders of Russia and China. They also have an arsenal of crowd control hardware at their disposal including water cannon and several "sound cannons" -- giant vehicle-mounted speakers that blast ear-piercing shrieks at unruly crowds. Roads near the convention centre in Queensland's state capital bristle with barricades and checkpoints, while part of the nearby Brisbane river has been sealed off to prevent the risk of a waterborne attack.
The G20's high profile means it has long been a magnet for anti-capitalist protests.
The last major G20 event held in Australia, in Melbourne in 2006, saw running battles between police and demonstrators, culminating in baton-wielding officers in riot gear fighting off an attempt to storm the summit venue. Since then, clashes have erupted at G20 meetings in Toronto, London and Pittsburgh.
Queensland assistant police commissioner Katarina Carroll said the unprecedented measures, two years in the planning, would allow swift and decisive action if needed.
More than 6,000 police officers have locked down Brisbane
"One of the greatest challenges faced by both protesters and Queensland police is that of lawful and peaceful protesters being infiltrated by violent and destructive elements," she told reporters.
"Protesters need to be aware of this and remain vigilant." Another concern for authorities is the threat of an extremist attack from jihadists inspired by the Islamic State (IS) group.
Australia lifted its terror threat level to "high" in September, just before raids in Brisbane and Sydney resulted in the arrest of an extremist group that allegedly planned to randomly behead a member of the public in order to demonstrate IS's reach.