The Olympics-themed parade runs from Piccadilly to Trafalgar Square where there is live music.
High-jumper Deirdre Ryan and boxer Kenneth Egan, who have qualified for the Games, are among those joining.
There are 220,488 Irish people living in London according to the last census.
Mayor of London Boris Johnson said: 'I'm delighted to once again be backing London's St Patrick's Day celebrations.
"That they are amongst the biggest outside Ireland reflects the longstanding contribution of many generations of Irish people to London life."
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More than a tenth of the Piccadilly Line train fleet is out of action because of emergency repairs to damaged wheels.
Rows of students stood and turned their backs before filing out of the lecture theatre.
Effie the endangered Western lowland gorilla has given birth to a healthy baby boy at London Zoo.