King Crimson made their breakthrough live debut by playing the Rolling Stones’ free concert at Hyde Park.
The concert took place in London’s Hyde Park on 5 July 1969. The Rolling Stones headlined the free outdoor event, which also featured King Crimson, Family, the Third Ear Band, Alexis Korner’s New Church, Screw and the Battered Ornaments.
For King Crimson, the appearance was a major early opportunity to perform before a huge audience, even though the group had formed only months earlier. Their debut album, In the Court of the Crimson King, followed in October 1969 and helped establish them as one of progressive rock’s defining bands.
A common mix-up is to describe the event simply as a Rolling Stones concert. It was indeed organized around the Stones’ headline appearance, but King Crimson’s participation is the key detail in this question. Estimates place the crowd somewhere between 250,000 and 500,000 people.