A huge mechanical elephant weighing 42 tonnes is due to thunder through central London, closing roads and drawing thousands of spectators.
The event began with a spaceship landing in Waterloo Place
The 40ft elephant will start parading through streets on Friday, part of a four-day street theatre show by French arts company Royal de Luxe.
The show began on Thursday morning with the appearance of a crashed spaceship in Waterloo Place, off Pall Mall.
"Most people have been intrigued by what's going on," said organisers.
It has taken two years to negotiate with all the authorities involved to bring The Sultan's Elephant show to London.
The elephant will make its first appearance on Friday
The show tells the story of a sultan from "far off lands" and his magical time travelling mechanical elephant.
The elephant is made up of hundreds of moving parts and is made largely of wood. It will thunder through streets, squares and public spaces, along with scores of performers and some large-scale puppets.
Nikki Webb, of Artichoke, which is putting on the event, said the spaceship structure had already caused a stir with Londoners.
"I think they have been amazed and there have been plenty of open mouths - one man nearly drove into a lamppost. Most people have been intrigued by what's going on...It's a piece of magic."
The show is being funded by the Arts Council England and the London Development Agency, with support from some private funders.
Traffic is expected to come to a standstill and roads will be closed as the elephant passes through.
Jean Luc Courcoult, who set up Royal de Luxe nearly 30 years ago, said putting on a show in London was difficult because they had to seek permission from so many people.
He told BBC London: "It can be really annoying for the city, but at the same time it's a real excitement for the audience.
"One of the main things in art in general is to create sights and images that remain printed in people's minds."
Police advise motorists not to use the routes around Trafalgar Square, Piccadilly and Regent Street over the weekend and some closures are also likely to be closed in The Mall and Horse Guards Road.