Tens of thousands of spectators watched muralists, painters, ceramicists, mosaic artists and others creating their works in a vast airplane hangar. Rivera painted his giant fresco “Marriage of the Artistic Expression of the North and of the South on This Continent,” currently displayed at the Roberts Gallery at the San Francisco Museum of Modern Art, at “Art in Action.”

The exhibition was the brainchild of prominent San Francisco architect Timothy Pflueger, one of six architects named to design the Fair’s core buildings.

As Therese Poletti writes in “Art Deco San Francisco: The Architecture of Timothy Pflueger,” the idea first came to Pflueger in 1929 when he observed sculptor Ralph Stackpole working on a giant statue while standing on a scaffold and hidden behind a screen. Curious passersby stopped to see what was happening behind the screen, giving Pflueger the idea of staging a public exhibition of artists at work.

“Art in Action” occurred because the 1939 World’s Fair was a financial bust. The exposition’s timing was terrible: intended to promote global peace and brotherhood, it opened in February 1939, when Hitler had already begun seizing parts of Europe and the Japanese Empire had invaded China.

On September 1, 1939, less than seven months after the Fair opened, Hitler invaded Poland, the beginning of World War II.

The Fair also had to compete with a rival World’s Fair in New York City. Even though San Francisco had just completed its two great bridges, it was still mired in the Great Depression.

Richard Reinhardt writes in “Treasure Island: San Francisco’s Exposition Years,” “The promoters had grossly overestimated the size and spending habits of the audience. They were expecting about 2,500,000 customers each month. Less than half that number showed up during each of the first two months.”

The Fair was forced to close six weeks early, in October 1939, with $4,166,000 in debt. To recoup some of the losses, a successful campaign was launched to restart the Exposition under the slogan “A Fair in Forty.”

The second incarnation of the Fair opened on May 25, 1940 — three weeks before Hitler’s troops marched into Paris.

To attract visitors and to replace the European masterpieces that had been featured in the 1939 Fair but had been shipped home, Pflueger convinced officials to sponsor the “Art in Action” exhibition. It would be held in the Palace of Fine and Decorative Arts building, a hangar built in 1938 to house Pan-Am’s Clippers, the famous “Flying Boats.”

The star attraction of “Art in Action” was Rivera. Pflueger had become friends with the Mexican artist a decade earlier when Rivera had executed two major commissions in San Francisco.

Pflueger flew to Mexico in April 1940 to personally negotiate the contract, but getting Rivera to San Francisco proved to be an arduous ordeal. He was caught in a murderous swirl of Stalinist violence and Mexican revolutionary politics and had to flee the country.

The day before the 1940 Fair opened, a 20-man Stalinist hit team that included Mexican muralist David Siqueiros made an assassination attempt on exiled Communist leader Leon Trotsky. Trotsky had been a close friend of Rivera’s and stayed at his house until the artist broke with him over Trotsky’s affair with Rivera’s wife, Frida Kahlo. The gunshots would have been audible at Rivera’s nearby house.

Terrified that the Stalinists would try to kill him next, he fled his home with the help of Hollywood movie star Paulette Goddard and young Hungarian artist Irene de Bohus (both of whom he was sleeping with) and went into hiding.

Pflueger had the U.S. consul arrange to covertly fly Rivera out of Mexico. He arrived in San Francisco on June 1 with only the clothes on his back.

Rivera moved into an apartment on Telegraph Hill and began working on his mural in early June. After a Stalinist assassin killed Trotsky in July, an armed guard was stationed at the bottom of the scaffold. (The precaution proved unnecessary.)

“Art in Action” received a glowing write-up on June 2 from San Francisco Chronicle art critic Alfred Frankenstein.

“Most unparalleled, of course, is the huge hall in the center of the building given over to the Art in Action section,” Frankenstein wrote. “Here the motto is exactly that which has been printed on cards and posted on the walls in the Fair’s press headquarters — ‘Genius at work.’ The foremost live genius, the bright star, the Venus and Raphael Madonna of this 20-ring circus of art is Diego Rivera.”

But if Frankenstein singled out Rivera as the main attraction, he noted that “many other artists are already on the job. Dudley Carter is hewing wood sculpture with his double-bitted ax, the Breuers are weaving, Helen Forbes is painting murals, Fred Olmsted is hacking granite into a likeness of Leonard da Vinci, and the potter’s wheel is in expert hands.”

By the time it closed with the rest of the Fair on Sept. 29, 1940, it had drawn tens of thousands of visitors, and 68 different artists had participated.

“Art in Action has thoroughly justified itself as the most important experiment in exhibition technique made in recent years, for it has served to break down the barrier between artist and public, to show the public the problems the artist has to solve and how he solves them,” Frankenstein wrote.

Rivera was not finished with his fresco when “Art in Action” closed, and he and his assistants continued to work on it for two more months. On Dec. 2, when the public was invited to see the finished work, an estimated 25,000-30,000 people attended.

Rivera’s mural, by far the most famous work created at the Fair, languished in storage for years before being displayed at City College and is now on temporary display at SFMOMA.

When it returns to a new performing arts center at City College in the next few years, it will join other works created at the exhibition, including Dudley Carter’s “Ram” (City College’s mascot) and several of Herman Volz’s mosaic murals — enduring legacies of a unique public exhibition dedicated to pulling the curtain away from the actual work of artistic creation.