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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The eagle-eyed resident spotted the mistake on signs just feet apart in south London.
The victim was attacked after bumping into a man at the ticket barrier at Putney Station.
The victim, who is his 40s, was found near the edge of palace grounds.