Forty years ago, Jethro Tull played an apocalyptic show at Red Rocks Amphitheatre amid tear gas, unruly crowds, hurled rocks, violent police officers and a swooping police helicopter.

It was to be the end of rock ‘n’ roll at Red Rocks forever. And for the five years after the concert, there was no rock music at the legendary mountain amphitheater.

“It was an overreaction by the police at the time, who had helicopters in the air,” Tull frontman Ian Anderson said recently from his office in southwest England. “We charged through police roadblocks, and I ran straight onto the stage and talked to the audience. (The police) knew there would be a full-scale riot if they arrested me.”

Anderson laughs about it now, calling it “a Top 10 strange/weird moment” while discussing his band’s Red Rocks date, scheduled for Wednesday, only two days shy of that fateful show’s 40th anniversary.

It’s fitting that Tull is playing Red Rocks on this tour, also the 40th anniversary of the record they were touring then, “Aqualung.”

So what exactly happened that night in June 1971? From 1,000 to 2,000 fans showed up without tickets to the sold-out concert, and they were directed by Denver police to a side of the mountain where they could watch the show. Some stayed there. Others climbed a wall into the venue. Others charged the gates en masse.

Back-up officers were called, and police chief George Seaton came out in the helicopter and dropped tear gas on the unruly masses himself. But the gas spread into the amphitheater, where Livingston Taylor was opening the concert, and suddenly a bad situation got worse.

“Backstage looked like an aid station, with doctors and patients sprawled out everywhere,” remembered retired promoter Barry Fey. “Boy, did I (mess) up. I didn’t realize how big they were. We should have done two shows.”

It’s hard to imagine now, but rock music wasn’t always welcome at the Morrison amphitheater. It was already a hard sell in city-owned venues, Red Rocks included, because of a violence at an Iron Butterfly concert at the Auditorium Arena in the late ’60s. But Fey had specially petitioned for the Tull concert, and he got it.

“Barry learned the hard way that you have to get three or four nights with an artist like that,” said Jerry Kennedy, who was a captain with the Denver Police Department in 1971 and later a division chief.

“I was running the police up there, and the place was under assault by thousands of people who wanted to get in. They decided they were going to rush the place, and that’s what caused the battle.

“They were throwing rocks. And I didn’t see it, but I heard that some of the officers were throwing rocks back at them. It was the first real incident of that kind that I’d seen.”

Amid all of this, Tull was devising a way to enter the amphitheater, which had been blockaded by the police. Anderson remembers charging through the police barricade and knowing that he was the only person who could calm the capacity crowd — which was swimming in tear gas at the moment.

“(The police) tried to turn us back and say, ‘You’re not allowed to go up there,’ so we just charged the gate,” Anderson said. “We jumped out of the cars and ran straight on stage to talk to the audience.

“It was like the Russians putting a flag on the ocean bed under the Arctic ice. Once you’ve done it, staked the claim, it’s tough to dislodge you. Once I was on stage in front of a microphone, they cops realized that they had to stand back.”

Anderson soothed the crowd and told them they were going to get a full set of music. He told them to put clothing over their mouths, and he encouraged parents with babies and small children to come to the apron so they could access the makeshift hospital set up backstage.

An iconic moment is forged

“People were passing babies down through the audience,” Anderson said.

It was a mess of an evening. But like Woodstock and Altamont before it, the concert was also a snapshot of America as it formed its relationship with rock ‘n’ roll.

“Back in the early ’70s, they didn’t know how to cope with rock concerts and rock people,” Anderson said. “The big production and how the audience behaved. . . . People now have more understanding, and civil and social savvy. They have an awareness about what it all is.”

Exiting the amphitheater proved to be as difficult as entering for the band, Anderson remembered.

“(The police) tried to get us on the way back down,” he said. “They were looking for us, but we were hidden under blankets in the back of a station wagon. They didn’t find us, and we got out of town.”

But the band left a trail of controversy.

Police chief Seaton recommended a ban on rock concerts at Red Rocks. Mayor William McNichols said there wouldn’t be rock shows there as long as he was mayor. And even Fey agreed, telling The Denver Post at the time that he wouldn’t throw any more rock concerts there.

Of course, that didn’t last long.

Fey sued the city in 1975, and a U.S. Circuit Court judge ruled in his favor. As Fey remembers it, the judge put this question to Denver leaders: “Who do you think you are, czars? You’re going to tell the people what they should listen to?”

Rock returned to the Rocks in ’75, and Fey’s popular “Summer of Stars” found its start in ’76.

“We went on to do hundreds of concerts there without a lick of problems because (Barry) got a handle on the problem,” retired division chief Kennedy said. “But in terms of problems, I encountered at venues over my career, that ranks in the top two or three.”

And it would have been worse if Tull had taken advantage of the act-of-God clause in its contract.

“The clause says that if anything crazy happens beyond the control of the band, they have the right not to play,” Fey said. “But he did play, and he played a full set.

“Ian Anderson is still my hero to this day. He went up there and hopped around with his flute and actually played a full set in the middle of the tear gas, in the middle of everything.”

Ricardo Baca: 303-954-1394 or rbaca@denverpost.com; @RVRB on Twitter

Jethro Tull

Flute-driven rock, 40th anniversary of “Aqualung.” Red Rocks Amphitheatre, Morrison. 7:30 p.m. Wednesday. $60.50-$97.50 ticketmaster.com.