Ballet fans Flock to Trafalgar Square

They provided the lights, the music and the stage, such as it was: the public provided the rest.

Instead of going at vast expense to see the Royal Ballet at the Royal Opera House, would-be Baryshnikovs and aspiring Fonteyns were able to have their own moment in the spotlight right under Nelson's Column.

And, for a magical moment, Trafalgar Square was full of arabesques and pirouettes - some, it has to be admitted, rather more rusty than others - all bathed in an eery light as ballet lovers and curious passers-by all took their turns at the DIY dance corps.

The installation, Flock, was the creation of choreographer Top Sapsford, who knows Swan Lake from years of dancing in the Royal Ballet's version, and digital media artists KMA (also known as Kit Monkman and Tom Wexler).

Their idea was to turn the square into an "enchanted space" where "ghostly projections" would create a magical "swan world".

Vivienne Gaskin, until recently the ICA's director of performing arts and digital media, was inspired to stage the project after seeing KMA's piece entitled Eng-er-land, which flooded the stage with projections of a virtual city that shifted in response to the movements of dancers.

Sapsford said recently: "I've worked a lot with digital films but this is the first time that I've worked on something with such a level of interaction. I love the idea of making the theatrical experience somehow deeper."

A light projector was set up, along with a thermal imaging camera which registered people's heat signals and triggered a spotlight which shadowed the dancers, filling the space with light projections that followed in their wake.

The more people who came into the space, the more lights there would be and - or so they promised, but it was hard to tell - the louder the music

would get. Under a perfect, cloudless sky, it was just about possible to imagine that the floodlit backdrop of the National Gallery was the Prince's Castle, and the fountains the lake of tears where the Prince finds the Swan Queen Odette, a beautiful maiden turned into a swan by an evil spell.

As for the dancers, it would have taken a rather larger leap of the imagination to confuse them with refugees from Covent Garden's corps de ballet.

They posed, they twirled, they struck attitudes: some looked as if they were really rather enjoying themselves.

Ed from Brixton said he loved the music - well, you can't get enough of that Tchaikovsky, can you? - but Jo from West Hampstead said that, considering it was based on Swan Lake, there should have been more swans. It was hard not to disagree.

Until tomorrow. www.ica.org.uk