New York Mayor Michael Bloomberg on Tuesday launched a competition among European cities aimed at inspiring new ideas to improve city life -- and was presented with a London Underground sign bearing his name.

Speaking at London City Hall alongside London mayor Boris Johnson, and joined by the mayors of Florence in Italy and Warsaw, Bloomberg said he was looking for "cutting-edge ideas" from Europe.

To mark Bloomberg's imminent departure from office, Johnson gave the 71-year-old tycoon one of the distinctive London Underground station signs emblazoned with his name.

Johnson, who is in his second four-year term, said Bloomberg's lengthy tenure was "a feat I think is unlikely to happen over here".

The Mayors Challenge competition -- started last year in the United States -- comes with a five million euro ($6.7 million) prize for the winner, with one million euros ($1.3 million) each going to four other cities that come up with the boldest ideas.

"We're bringing it across the Atlantic to find a whole new set of cutting-edge ideas," said Bloomberg.

"We're looking for the most visionary and transferable ideas out there.

"European cities really are among the world's most innovative."

Rejecting the idea that philanthropy was essentially for vanity, Bloomberg said private donations could support "things that are new, innovative and out of the box," citing the 19th-century backing that sponsored impressionist art.

Johnson complained that wealthy Britons did not share the inclination of their US counterparts to pump their resources into helping wider society.

"The trouble with people in this country, what they really want to do when they've amassed colossal wealth," he said, was to buy as big a property as possible and guard their money "in some desperate way".

"There is still something in Britain that regards giving on a huge scale as being somehow ostentatious. That is ridiculous.

"The sooner people get over their lust for buying great 'schlosses'... the better."

Johnson said while the British capital would enter the competition, "if London fails to win, we will have no shame in nicking (stealing) the idea anyway."

Bloomberg completes his third four-year term at the end of December. He said besides his business and philanthropic interests, he would continue to work on the US "gun problem."

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