A big part of the thrill of rock concerts is being immersed in the crowd, shoulder-to-shoulder with hundreds of other fans while your favorite band performs your favorite songs.

Or, in some cases, hundreds of thousands.

Some rock concerts have drawn record-breaking numbers of attendees, often in spaces that frankly aren't designed or equipped to host that many people. The reality is that this can sometimes be dangerous, and especially in situations where the concert in question is free, there's little anyone can do to deter people from coming. In the best of circumstances though, these large crowds behave perfectly well and everyone involved has a good time.

It can be unnerving for the artist, without a doubt. When asked how he felt about playing in front of 131,000 people in Sao Paulo, Brazil in 1981, Freddie Mercury quipped: "Haven't done it yet!" (He would later recall the concert fondly, telling Japan's Music Life magazine: "It was truly wonderful, and even we [Queen, who had never played in Brazil prior to that] were unexpectedly surprised. Even if we do our regular routine, how they feel it in their bodies is a completely new thing. There are no rules about how to feel our music, so their reactions were totally natural."

READ MORE: Marathon Concerts: Nine of Rock's Most Legendary Long Shows

Below we're taking a look at 20 of the Biggest Single-Act Rock Concerts Ever, in order of size. For the purposes of this list, we're excluding multi-act festivals — Woodstock, for example — and have stuck with shows billed specifically as one band or artist.

20 of the Biggest Single-Act Rock Concerts Ever

Hundreds of thousands of people witnessed these historic shows.

Gallery Credit: Allison Rapp

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