About 55,000 spectators are expected to line the streets of London to for the world's most popular triathlon.
About 14,500 athletes - including Formula One world champion Jenson Button - will first swim near the ExCel Centre in east London's Docklands, then take to their bikes before a final run.
Advanced entrants can take the extended cycle course to Tower Bridge, taking in views of Big Ben and the London Eye.
The event is in its 14th year and raises about £3m for charity.
It is aimed at all levels of expertise from the super sprint category, with a distance of 8 miles (13km), to the 32-mile (51.5km) Olympic section.
Organisers have built a pedestrian bridge on the east side of the ExCel Centre to allow spectators a good view of the dockside front.
The walkways from the Docklands Light Railway stations at Custom House and Prince Regent are recommended for those wanting to watch the bike race.