When the harsh realities of 1968 derailed the pic he planned to make, Haskell Wexler decided to hold a mirror to America’s turmoil with “Medium Cool.”

It was a scene that surely never will be replicated.

A crew is standing by to start shooting a movie. The script is undergoing a final polish. The cast is at the ready. The studio OKs the budget.

And then the director summons everyone to a meeting to make his surprise announcement. He intends to start on schedule — but he’s going to shoot a different script and a different movie. Vastly different.

That was the scene that took place exactly 35 years ago this month. The director was Haskell Wexler, who at the time was a brilliant young cinematographer about to start his first studio film — but not the studio film that had been assigned him.

His “substitute” movie was “Medium Cool,” which in time was to become a cult classic.

A superb exercise in cinema verite, “Medium Cool” captured the mayhem of Chicago 1968, when the Democratic National Convention exploded in riots and the very fabric of society seemed to be shredding.

“Medium Cool” was the product of a string of apocalyptic events: Martin Luther King and Bobby Kennedy had just been assassinated. Race riots were wracking the country along with Vietnam protests. Lyndon Johnson suddenly withdrew from the presidential race. Even pop music seemed torn between social upheaval and psychedelic rapture.

The nightmare times radicalized Wexler, who’d been signed by Paramount Pictures to shoot an urban melodrama called “Concrete Wilderness.” The low-budget film was to be filmed in Chicago on a negative-pickup deal.

But when Wexler arrived on location, the sheer bedlam immobilized him. He phoned a friend at the studio, who happened to be me, and explained his dilemma. “How can I shoot this nice little melodrama when chaos is exploding all around me?’ he demanded. “I can put together a new script. I have my cast and crew. I know what I have to do.”

And he did it.

The film he shot was crude, its narrative bumpy, but it superbly captured the moment. There were the phalanxes of cops and National Guardsmen hammering the bizarre assortment of hippies and yippies and peace activists. The teargas canisters were exploding everywhere. Machine-gun-toting troopers slammed into the crowds. The whole nation seemed to be imploding.

Looking back on it today, Wexler is still amazed and appalled. “I am a very patriotic person,” he reflects. “But at that moment in time I felt like we were all enemies within our own country. It was as though the ruling powers were saying, ‘This is America and you’re not part of it.’ ”

Wexler’s movie was strong stuff. Too strong for Paramount Pictures, as it turned out. When the film was finished, some powerful voices on the board of directors of the parent company, then called Gulf+Western, demanded that “Medium Cool” be shelved.

The Democratic Party, in their view, was already in shreds as a result of the convention and would suffer still further damage from the film. When their censorship efforts were stymied, they summoned up a list of technical obstacles. A nude scene in the movie would earn it an obligatory X. Clearances had not been obtained from many of those shown in the rioting.

Ultimately, the movie was released, but it was a token effort. Major festivals were inhibited from playing the film. The campaign was meager.

“Medium Cool,” like another film of that moment, “Harold and Maude,” ultimately was “discovered” by a substantial audience, but it was a slow, painful process. It is still admired today at festivals and film schools and represents a truly courageous example of gonzo filmmaking.

Arguably the only derivative moment in the film occurs at the end, when Wexler points his camera at the audience — a trick borrowed from Godard. It was as though he were telling his audience: This is fiction but it is also true. And it is your truth, too.

Now that reality TV holds a grip on the ratings, it would be a healthy antidote for audiences to get a dose of the real reality.

For the truth, too, can be cool. At least, “Medium Cool.”