Tunick, famous for photographing crowds of nude people around the world, said the May 6 shoot could be bigger than one he did in 2003 with 7,000 volunteer models in Barcelona, Spain.
"This could be my largest work ever," Tunick told a news conference. "We're really hoping that all eyes will be on Mexico City on May 6 because this could be ... bigger than Barcelona."
The historic Zocalo plaza measures about 21,000 square yards — the size of about five football fields — and can fit up to 85,000 people standing.
Tunick said last month that 3,700 participants had registered for the shoot. On Sunday, he said more were signing up but declined to give a figure.
The Brooklyn, N.Y. artist said he had all the necessary permits and the surrounding streets would be closed to traffic for the morning shoot.
"I'm very honored and pleased to announce that my art installation would be at one of the nicest and oldest places in the world, a place of great architecture uniformity," Tunick said.
The square, in one form or another, has been at the heart of Mexico City since it was founded by the Aztecs in 1325. When the Spanish conquered the city in 1521, they demolished the Aztec temples and palaces around the plaza and gave it much of its present form.
Earlier this month, Tunick said he wanted to do the shoot at the pre-Aztec ruins of Teotihuacan, north of the capital, but that he was also considering other locations.
Tunick said the concept of the plaza shoot would be to re-create or pay tribute to the lakes that once covered much of the Mexico valley.
"For some mysterious reason, a wonderful reason, the Mexican citizens are signing up in large numbers," he said. "The waves and the nature of the place will have the purest, natural thing in the world, which is the human body."