Hundreds of youngsters are taking part in a huge procession of Calypso dancers and floats for Children's Day, dancing around a route between Great Western Road and Ladbroke Grove.
Bank Holiday Monday sees the adults take to the stage with the Grand Finale parade. The event is considered one of the largest street festivals in Europe.
Transport for London said some road and Tube closures would be in place.
William Medley, 35, took his three-year-old son to the event and said it had a "fantastic party atmosphere".
He said: "It's an opportunity for kids to run around and see something different, interact with different people and experience the food, smells, sounds and dancing."
Families made the most of the dry weather by watching the procession as West Indian flags and Union Jacks were draped from balconies.
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This year, for the first time, the Metropolitan Police have introduced a dispersal order around the event to minimise the chance of trouble or disruption.
Supt Robyn Williams said: "Basically anyone who is committing anti-social behaviour, or is likely to, will be asked to leave and will not even be allowed in.
"We've taken those measures based on the intelligence and the history and to actually support the residents."
The event this year has come in for criticism after demanding journalists and photographers pay Â£100 for accreditation and share their work on the festival's website.
But organisers said it was necessary to meet the requirements of a modern sustainable carnival.
There is also some discrepancy over the year the carnival officially started.
The Notting Hill Carnival Enterprises Trust said some people thought the first carnival dated back to 1964 while others argue it started two years later in 1966.
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Crowds of people have descended on west London for Notting Hill Carnival.