The free events are being organised in partnership with the Council of Irish Counties Associations and the St Patrick's Day advisory forum.
The parade from Piccadilly to Whitehall will set off by Green Park at around midday, with floats, musicians and community organisations taking part. A highlight will be the appearance of The Puca, a 36 metre-long inflatable dragon created by Bui Bolg - a team of skilled artists, engineers, performers and designers led by artistic director Colm Lowney.
New to the Trafalgar Square festival will be comedy and film tents. The London Irish Comedy Festival will host the comedy space.
Revellers can also explore an Irish food market, and watch contemporary and traditional music, Irish dancing and a selection of short films on the main stage.
The Mayor of London Boris Johnson said: "I am delighted to once again be supporting London's St Patrick's Day Festival and parade. Amongst the biggest outside Ireland they are a fantastic celebration for Londoners and visitors alike, as well as being a tribute to the contribution that generations of Irish people have made to life in the capital."
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