Parade groups and floats representing the Irish counties including bands from Ireland and the UK will be taking part in the festival in Trafalgar Square.

The water in the square's fountains will be dyed green for the occasion.

The parade begins at 1200 GMT from Hyde Park Corner and then via Piccadilly, Regent Street, Trafalgar Square, ending at Whitehall Place.

'Enormous contribution'

The festival will include dance and music workshops, a children's play area, Irish arts and craft and exhibitions.

There will also be an Irish food market in Covent Garden.

Mayor of London Ken Livingstone said: "By celebrating St Patrick's Day we are marking the enormous contribution that generations of Irish people have made and continue to make to the social, economic and cultural life of our city.

"I'm particularly pleased that the festival will take place in favourite London squares."

The festival, which ends at 1800 GMT, was last held in the Square in 2002 when it was launched.