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."
More top news
The iconic Cold War jet is due to be grounded later this year on safety grounds.
Yousaf Khan and Xavier Christopher took their young victim to a secluded alleyway in Northholt where they also urinated on him.
After a wet and windy start to the week, conditions will gradually improve, with temperatures creeping up, and up.