“We were just exploring and finding out what could be done that was different from what everybody else was doing. King Crimson was probably the first of progressive rock music. Was it? Well, you tell me. I don’t care, actually, if we were the first, second or third. But I think it did make a pretty major statement coming from nowhere.”

– Michael Giles, interview with the author, 2013

Open your mind, relax awhile and float downstream. Picture yourself in London’s Hyde Park on a sunny summer afternoon – Saturday, July 5, 1969, to be precise – sitting on the grass and soaking up the rays as part of a crowd who are there to see The Rolling Stones. It was the group’s first concert in a couple of years and an emotional occasion, taking place only two days after the death of former guitarist and founder member Brian Jones, who had been replaced by Mick Taylor. The night before there had been a candlelit vigil for Jones in the park and by show time, between 250,000 and 500,000 fans had gathered in anticipation.

The Stones were rather ragged, with tuning problems, but something epochal had happened earlier on in proceedings. It was a typically eclectic late-Sixties bill, featuring the acoustic, oboe-led urban ragas of the Third Ear Band, the blues of Alexis Korner’s New Church, Family, The Battered Ornaments, and Screw, who quickly disappeared into obscurity – but at least they played to a gathering that few other groups could even dream about. But the group the cognoscenti were particularly looking forward to were King Crimson.

They had only been in existence since January, but had already caused a stir on the London gig circuit. Concurrently they had invited journalists and record company representatives over for salons in their rehearsal studio in the basement of the Fulham Palace Cafe in west London. In March an impressed Simon Stable of International Times likened them to a cross between Family, The Moody Blues and The Pretty Things.

King Crimson’s first concert had taken place at the Speakeasy in London on April 9 and guitarist Robert Fripp, an assiduous diarist, mentions in his entry of that day: “Massive Success, the word starts to creep about in the business”. The Moody Blues had come to see them rehearsing in March, but a mooted tour support slot was abandoned. Fripp had it on very good authority – from one of the group members via manager David Enthoven – that The Moody Blues had considered King Crimson “too strong” for a support. They almost certainly would have been.

At Hyde Park, Mick Jagger opened a box full of butterflies in memory of Brian Jones. King Crimson let loose something else entirely. Rather than delivering some soothing sounds to waft across the long-haired crowd, or some groovy blues-rock to get heads nodding, King Crimson’s set roared off into the concentrated aggression of "21st Century Schizoid Man", all acute, jagged lines, with a fiercely swinging, almost bebop instrumental section and a breathtaking, high-velocity unison passage, the like of which no one would have heard before.

The set crackled with energy and intensity. Dramatic Mellotron-led ballads, not so far removed from some of The Moody Blues’ songs, were played alongside a cover of Donovan’s “Get Thy Bearings”. The song’s hippieish exhortation for us all to “get stoned” prompted the flashing of a few peace signs, but Crimson’s version was cut with episodes of free improvisation remarkable for a rock group, particularly Ian McDonald’s torrential sax solo, which stretched the song to breaking point. No “new band” had played like this before to such a big audience. In terms of exposure and impact, a showcase at such a high-profile event was a godsend.

New Musical Express journalist and Stones fan Nick Kent has these recollections of their set: “They blew The Rolling Stones off the stage. King Crimson came on and they were just fucking amazing. They were seriously good at what they did.” If fate had not dealt such a generous hand to King Crimson, who were still to release a record, they might have been playing that Saturday in some small, smoky, sticky-floored provincial dive in the back of beyond to thirty people and the proverbial dog. But how did it feel playing to, depending on estimates, around half a million people?

Drummer Michael Giles remembers it this way: “When you are onstage you can’t hear much as the open air sucks the sound away. You don’t get any reflections like you do in indoor venues. It’s a lot easier to play to a huge number of people, as it’s just a sea of faces and very impersonal: it enables you to just get on and do it. But if you are in a small club and someone is a few feet away looking at you it can be quite intimidating.”

Alongside the hippie homilies of cover versions like “Get Thy Bearings”, there was an aspect of the group’s music that was, crucially, far removed from the benevolent, dreamy themes of English psychedelia that had been prevalent a year or two before. There was something about it that was hard-edged and forbidding. And at times quite frightening. “King Crimson, is it benign? Yes,” says guitarist Robert Fripp now. “What is scary about King Crimson is that there is something real about and when we come up against something real, a real intelligence a real presence, it can be overwhelming.”

But some contemporary accounts of the group’s live shows found little that felt benign, with the correspondents, in the parlance of the day, being somewhat “bummed out” by King Crimson’s music. Melody Maker’s Alan Lewis wrote: “They created an almost overpowering atmosphere of power and evil.”

This was mainly in reference to the set closer, a version of Gustav Holst’s “Mars, The Bringer Of War” from his orchestral suite, The Planets (1916). The crowd at Hyde Park heard just a few of minutes of this, but in concert, it would typically stretch out to ten minutes or more. King Crimson’s version of this piece was one of the first and best classical adaptions of the whole progressive rock era. They took the main “riff ”, the ominous backbone of the piece in 5/4, decelerated it slightly and repeated it in the most single-mindedly brutal way, with McDonald’s Mellotron playing the upwelling melody line and gradually cutting loose into discordant chaos. One can imagine that any audience members who had decided to illegally expand their minds before or during the show would have been sweating, hanging on, hoping it would end soon.

King Crimson also had the power to intimidate their fellow groups. Less than a week after Hyde Park they shared a bill with Gracious, who were still in the process of finding their collective voice. Their drummer Robert Lipson recalls the occasion:

“We did a double bill with King Crimson at the Beckenham Mistrale Club,” he recalls. “Robert Fripp came into our dressing room and said, ‘We want to do the middle section, can you do the first bit and you can close it?’ We said, ‘Yeah.’ So we go and play our three-minute nice little pop songs. They came on and did “21st Century Schizoid Man”, and that was the end of that. I’m not kidding. I don’t think I have ever been so blown away by a band ever. Fuck! Not only can they play, but what are these songs? And the timing?”

Gracious guitarist Alan Cowderoy adds: “Their performance was so extraordinarily good that we made some feeble excuse that water had been spilt on our plug board and we couldn’t play the second set.”

A New Day Yesterday: UK Progressive Rock & The 1970s is on sale now exclusively via from The Wire bookshop. Subscribers can read Dave Segal's review of the book in The Wire 342.