The two machines will bore their way from Royal Oak near Paddington to Farringdon station as part of the £14 billion project to create a new underground route for Crossrail.
At Farringdon, east-west Crossrail will provide a connection with the revamped north-south Thameslink rail route.
The ceremony at Westbourne Grove was attended by Transport Secretary Justine Greening and London Mayor Boris Johnson although the planning of Crossrail took place long before either of them took office.
More than 3,000 people are now working on Crossrail which is Europe’s largest construction project.
Later this year Phyllis and Ada will be augmented by other machines, Victoria and Elizabeth, and Mary and Sophia, which will bore other sections of the Crossrail route.
Phyllis was named after Phyllis Pearsall, who devised the original 1935 London A-Z map, and Ada Lovelace, a 19th pioneer of computing.
Richard Coackley, president of the Institution of Civil Engineers, said: "This project is a feat of engineering – giant 1,000 tonne machines each 150 metres long carving 21 kilometres of twin tunnels under London, with the ultimate aim of improving quality of life and boosting the London economy."
Railfuture has spent nearly 40 years campaigning for Crossrail. The project was shelved by unlamented Railtrack following privatisation and was nearly abandoned again by the coalition government in 2010.
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