Roads were already busy on Sunday (April 9) from 10.15am to celebrate the Sikh New Year.
The Vaisakhi parade, which took place in hot, sunny conditions, is thought to be one of the largest in Europe.
Sadiq Khan attended in what was believed to have been the first ever visit from a London Mayor, according to Sikhs for Labour.
The London mayor said: "Sun is shining here in Southall at the Sri Guru Singh Sabha Southall Vaisakhi parade, one of the largest in Europe.
"London is open.
"Londoners don’t just tolerate our diversity, we celebrate it. Brilliant to be back in Southall alongside thousands at Vaisakhi celebrations."
Last year the mayor attended alongside Conservative mayoral candidate Zac Goldsmith before the election.
Those in attendance started the day at Havelock Road Gurdwara before finishing at the Park Avenue Gurdwara.
All roads on the procession route were closed from around 1pm, which meant bus routes into through the area were diverted.
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