"London is a city that I have a special love for"

The poster reads simply "Imagine all the people living life in peace" - a lyric from Lennon's chart-topper and paean to non-violence, Imagine.

Former Beatle Lennon was murdered by a lone gunman 21 years ago, as the couple returned to their Manhattan apartment.

Simple message

Written in black type on a white background, the simplicity of the billboard is at odds with the array of dazzling displays nearby, which advertise fast food and fizzy drinks.

Ono has posted similar posters in New York's Times Square and Tokyo in recent months, and at the end of September took out a full page "advert" in the New York Times with the same message.

She has also rented a New York billboard space to offer Christmas greetings to her fellow New Yorkers.

Seen by thousands

"After the horrible events of 11 September, I thought it was a very important time to remind people of this message, because the world needs peace," she said.

The words of the poster come from Lennon's song Imagine

"I wanted it in London because it's a city that I have a special love for, and have fond memories of."

Ono's message will be seen by thousands of shoppers and tourists, who pass through Piccadilly Circus everyday.

But it is open to question how many younger people will know who is behind the message.

A few days ago, 20-year-old American pop diva Britney Spears highlighted the gap between her generation and older pop music fans when she revealed she didn't know who Yoko Ono was.