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
Police arrived at the 73-year-old's home after Alfie chased a delivery man.
Officers believe the three assaults are linked and are appealing for information about the suspect.
Nine addresses in Lewisham were searched and 13 arrests made early on Tuesday morning in a planned, intelligence-led operation.