Thousands of naked volunteers pose for U.S. photographer Spencer Tunick at Mexico City's Zocalo square May 6, 2007. A record 18,000 people took off their clothes to pose for Tunick on Sunday in Mexico City's Zocalo square, the heart of the ancient Aztec empire.

MEXICO CITY - More than 18,000 people stripped down and bared it all in Mexico City's vast main square Sunday for US photographer Spencer Tunick's biggest nude shoot yet.

Standing up to salute, crouching in fetal positions and lying prone on the tiles of the Zocalo plaza, the volunteers formed a sea of flesh that Tunick snapped from balconies and a small crane in the morning light.

"What a moment for the Mexican art scene!" Tunick said in a news conference. "I think all eyes are looking south from the United Sates to Mexico City to see how a country can be free and treat the naked body as art. Not as pornography or as a crime, but with happiness and caring."

The Brooklyn, N.Y., artist has become famous for photographing thousands of naked people in public settings worldwide, from London and Vienna to Buenos Aires and Buffalo.

But the Mexico City shoot dwarfed all others. Previously his best turnout had been 7,000 models in Barcelona in 2003.

"I just create shapes and forms with human bodies. It's an abstraction, it's a performance, it's an installation," Tunick said. "So I don't care how many people showed up. All I know is that I filled up my space."

The heart of this city since it was founded by the Aztecs in 1325, the Zocalo measures about 21,000 square yards ¡ª the size of five football fields.

Men and women from a broad cross section of ages and social classes began arriving before dawn, although most volunteers were young men.

"The important thing is not that it's your body or someone else's but that you participate in something as a society," said Oscar Roman Munoz, a 25-year-old engineer. "This reflects the need for change and integration in world trends."

For Tunick's first photo, the models stood upright and gave a military-like salute to their national flag. In another, they lay down to form a blanket of flesh around a naked man in a wheelchair. Between shots, they burst out into verses of Mexican folk songs such as "Cielito Lindo."

Public nudity is hardly a novelty in Mexico City, where protesters often march through the streets wearing only their underwear or nothing at all.