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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In a wide-ranging Peston on Sunday interview, he said Parliament security needs reviewing and the PM's counter-terror strategy is flawed.
The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.