The under-19 and under-23 athletes will compete first, with three days of racing beginning tomorrow.
Event media and communications manager Richard Gee said it was great to get the championships under way.
He said the best river to watch the competition on was the Tarawera River, which had been set up for spectators.
Racing will also be held on the Rangitaiki River near Kawerau and, for senior and masters events, the Kaituna River at Okere.
The feature day of the championships will be held next Saturday, November 23, on the Tarawera River, with viewing and events at Waterhouse St Reserve from 10am to 5pm.
All four of Kawerau's schools have taken part in the Adopt A Nation programme which runs in conjunction with the championships.
Students have spent weeks in the build-up to the event learning about rafting and the 27 countries attending the championships.
Kawerau District Council organiser for the event Lee Barton said students were really enjoying learning about the different countries.
"Students have learned an enormous amount about their adopted countries, from geography and language, all the way to making waffles in food technology."
Many of the children who took part in the Adopt A Nation programme met with athletes during the opening ceremony.
Athletes also gathered at Waiariki Institute of Technology in Rotorua yesterday evening as part of the opening day celebrations for the world champs.
Waiariki will be the hub for the world champs.
Today marks day two of official practice for the under-19 and under-23 teams.
Two countries, Iran and Argentina, have pulled out from the championships - meaning there are now 62 teams competing.
