Tunick, who has raised eyebrows by staging mass nude photo shoots in cities from Dusseldorf in Germany to Caracas in Venezuela, smashed his previous record of 7,000 volunteers set in 2003 in Barcelona, Spain.
Directing with a megaphone, Tunick shot a series of pictures with his Mexican models simultaneously raising their arms, then lying on their backs in the square, as well as another scene on a side street with volunteers arranged in the shape of an arrow.
One of the world's biggest and most imposing squares, the Zocalo is framed by a cathedral, city hall and the National Palace official seat of government, which is adorned with murals by Diego Rivera.
A ruined temple next to it was once the centre of the Aztec civilisation and was used for worship and human sacrifice. Spanish conquistadors used bricks from the temple to help build their own capital.
Some participants said the massive turnout showed that Mexicans, at least in the capital, were becoming less prudish.
Mexicans are not used to showing skin. Most men wear shorts only while on vacation, and women tend not to put on miniskirts because of unwanted whistles and stares.
"This event proves that really we're not such a conservative society anymore. We're freeing ourselves of taboos," said Fabiola Herrera, a 30-year-old university professor who volunteered to strip, along with her boyfriend.
The capital of the world's second-biggest Catholic nation, where tough-guy masculinity and family loyalty are held dear, has recently challenged some important traditions.
Last month, Mexico City legislators legalised abortion in defiance of criticism from church officials.
Also, gay couples are getting hitched in civil ceremonies thanks to recently passed laws in the capital, and lawmakers plan to debate whether to legalise euthanasia.
"They're losing dignity as men and women," said 63-year-old Armando Pineda, leaning against the cathedral and watching the now-dressed models leave the plaza. "It's an offence against the church."
The Mexico City metropolitan area is home to about 18 million people.