The director of the Museum of Modern Art, Glenn Lowry, earlier this year warned that if New York doesn’t invest in arts programming, it could lose its status as the cultural capital of the world.

Mr. Lowry said other cities are “incredibly hungry” to lure artists from New York and that “competition is truly fierce out there among cities.”

He singled out London as a prime rival of New York. “London took on in the early 1990s, on behalf of Britain, the idea of a Cool Britannia: Let’s make London the showcase for the millennium. And by using the lottery and investing billions of dollars in capital projects, they were able to actually create the impression that London’s the hot place to be,” Mr. Lowry said at a conference organized by the Center for an Urban Future, the Rockefeller Foundation, and the Partnership for New York City.

The London contemporary art scene is booming, what with the opening of the Tate Modern six years ago, the mega-collector Charles Saatchi’s various acquisitions, and the movement of Young British Artists that he helped spawn. Like MoMA, which just got a $425 million renovation, the Tate Modern, a converted power station, is about to get a $397 million expansion, and is expected to become even more of a force on the international contemporary circuit. Under the direction of Nicholas Serota, the Tate Modern is planning to expand photography, film, video, and other major areas of visual contemporary art.

Some say New York is still in a class by itself when it comes to the depth of art collection and the number of art galleries.

“I’m actually willing to go to London to try and poach on their turf,” Mr. Lowry said. “But you know what? There’s nothing to poach. It’s all over here.”

The director of the Gagosian Gallery in Manhattan’s Chelsea, Ealan Wingate, said the gallery’s New York and London locations are both extremely successful. Gagosian opened the largest commercial art gallery in London in 2004. At the time Mr. Saatchi said it could permanently change the London art world. And new galleries continue to open there, signaling that the city’s art scene has momentum.

Mr. Wingate said, however, there is something unique about opening a show in New York.

“It’s like opening up on Broadway. Yes, playing the West End is extraordinary, and I’m not going to deny the fact that there is fantastic London theater. Playing New York, Broadway has a different cachet. I think the same is true internationally for an artist, that the New York show is of special importance.” Mr. Wingate said.

The cities are linked by artistic cross-pollinations.

Mr. Saatchi’s “Sensation” exhibit, for example, which first appeared in London, made a huge splash at the Brooklyn Museum in 1999. In a few weeks, the British sculptor Anish Kapoor will unveil a nearly three-story-tall stainless steel installation at Rockefeller Center in Manhattan. The sculpture will be on display from September 19 to October 27. And on October 4, the Royal Academy in London is partnering with Mr. Saatchi’s gallery to open “USA Today,” a collection of American artists.

Londoners have seen a drastic change in the art landscape in the last 15 years. The once-working class East End is now bustling with more than 80 galleries and has been called the center of the European contemporary art scene with the famed White Cube gallery at its center.

The former director of exhibition for the Institute of Contemporary Arts in London, Matthew Higgs, said much of the growth and changing dynamic in London stems from the influx of young European artists. “London provides a perfect gateway into Europe and now a perfect midpoint between the emerging markets in Russia and China,” said Mr. Higgs, who is now the director of White Columns gallery in New York. He said that has given London a unique momentum and energy and given it a more international and cosmopolitan art community.

Part of the distinction revolves around the more open relationship the public has with art and the continued state funding for art programs. The highly competitive London newspaper and magazine market ensures that London publications follow the arts closely to win young readers. The annual fall Frieze Art Fair, begun in 2003, has become something of a craze.

Since the Tate Modern opened six years ago, it has attracted more than 22 million visitors. The museum — along with the construction of a pedestrian bridge, designed by architect Norman Foster, over the Thames River — has been credited with revitalizing the South Bank area into a bustling labyrinth of restaurants, cafes, and galleries.

The director of the Agency Contemporary, Bea de Souza, whose gallery is on the East End, said the area is the equivalent of “New York’s Chelsea.”

A whitewashed gallery with unfinished wood-panel floors, the gallery moved in 13 years ago “with the pigeons,” Ms. de Souza said.

When she first opened the gallery, the area had only one other and the two paid to shuttle clients in taxis between them. When the clients were done for the day, Ms. de Souza said, the galleries hired a cab to take them to across town to get a cup of coffee “because you couldn’t find one here.”

At the time average rent was just under $1,000. Now, $5,000 is a minimum rent, and the area has become so gentrified that artists have long moved studios elsewhere.

The American correspondent for Apollo, Louise Nicholson, said the Tate Modern is not only a success in its own right, but has also boosted arts activity across the board in London and increased the number of people interested in contemporary art.

She said Mr. Lowry is looking for “seminal art” for what is “probably the greatest museum of contemporary art.” That he said he can’t find those pieces in London does not mean that London does not enjoy a thriving contemporary art world, she said.

Other art experts say nothing comes close to New York’s Chelsea in terms of the number and caliber of galleries.

An art critic for The New York Sun and the editor of a Web site called artcritical.com, David Cohen, said: “Chelsea is basically the world’s biggest museum of contemporary art. London just doesn’t have anything that compares to that.”

Mr. Cohen, who lives in New York, but is British, said that while London has far more public funding for art and has become more of a player in the contemporary art, especially since the opening of the Tate, it is “not on the scale of New York.”

Ms. Nicholson said that London galleries are more spread out and that visitors must seek things out because of the geography.

Mr. Cohen and others said the Tate Modern’s 20th-century non-British art collection is thin, and New York has far more galleries and more wealthy collectors looking to buy.An official at the Tate Modern said that museum has launched a campaign to acquire more international contemporary art from outside Europe and North America.