Several enormous crowds on the National Mall in DC. Many estimated to exceed 1 million people. But both organizers and the National Park Service are notorious for mis-estimating for their own purposes.
:eek:
Woodstock (the original one).
Simon & Garfunkel concert in Central Park.
Bastille Day in Paris.
Gay/Lesbian marches in Washington: one had a million people.
I think the biggest was the centennial of the Statue of Liberty in 1986. A crowd of 2 million, jammed into the tip of Manhattan.
October 27, 2003. Monday Night Football game between San Diego Chargers and Miami Dolphins at Sun Devil Stadium in Tempe, Arizona.
The Poll Tax Riot, 1990, ~300,000 people.
Hong Kong Handover, 1997, millions.
Glastonbury Festival when the wall got broken down, 2000, ~250,000.
Chinese New Year, Hong Kong, 2005. ~500,000 people.
Cursa Corte Inglés the first year it was entering the still-being-built Olympic Stadium. Bloody well over 100K participants, plus onlookers… in previous years I’d taken about 1h (quite good for a walker) but that year the press of people was so horrible that I took over 2h. The loudspeakers kept saying “don’t force yourself, run at a comfortable pace” and I kept thinking “if only I could!” I finished dehydrated, with the shakes and dizzy. Never went again.
John Paul II’s visit to Xavier. Dunnow how many people, but the explanada was full and then some.
John Paul II’s Youth Meeting in Santiago. It wasn’t just the amount of people, it was from how many places. Trading songs with other groups was cool 
An anti-new-education-law protest in Madrid in the '80s. According to the government it was about 50K people; according to the police, about 100K; according to the organizers, about 250K.
Mardi Gras several times, Kentucky Derby infield several times, Indy 500, Monaco Grand Prix, New Years Times Square, Orange Bowl (BSC Championship), Bastille Day in Paris, Football game at the Big House in Ann Arbor, the celebration on the Champs after France beat Brazil in the World Cup last year, Queens 72nd (I think, it was a big one) birthday in Bangkok, and the Lichtenstein Day festivities where I met the ruling Prince.
Some anti-war rally in Hollywood, CA in the spring of 2004.
I was trying to go shopping with my sister and we got stuck in traffic for 4 hours because of those assholes 
120,000 at Knebworth 1976 with the Stones headlining…
And the Hogmanay celebrations in Edinburgh regularly used to draw well over 100,000 people until they introduced better safety precautions… not sure what the maximum I attended was - 150,000 or so I think.
Cal Jam 1, 1974 at Ontario Motor Speedway, ~200,000
Bi-Centenial 4th of July 1976 in Washington D.C. I have since sworn off any large crowds for any reason.
Same here. Not sure which was actually the biggest…
Clinton’s 1st inauguration was one…Anti-war, pro-choice rallies. Plus , since I work down near the mall, I get caught up in every !@#$% pissy demonstration known to man.
We were two and a half blocks from St. Paul’s for Charles and Diana’s wedding, crowd estimated at 2 million.
Biggest and most significant to me:
Game 3 of the '04 ALCS when we thought it was all over
Games 1 and 2 of the '04 World Series
Opening Day 2005 in Boston, when the banner was raised and the rings were awarded.
Biggest People-wise:
The Oct. 1, 1979 Papal Mass on Boston Common. Estimated at 400k people.
The baseball stuff was way better.
Probably the Breeder’s Cup at Arlington Park. That was in - er - 2002 I believe. About 50,000 people.
The 3rd of July in Chicago. Multiple years between 2000 and 2005. Well over 1,000,000.
The last time was pretty awful, though, so I’m never doing it again!
This makes you the bravest person I know!!! 
Visited planet Earth, at a time when its population was approaching (or perhaps exceeding) six billion!
Calgary Stampede - 1.26 million folks over the event duration. About 150,000 on average per day.
The Million Man March. Attendance was anywhere from 600,000 - 1mil, depending on who you believe. My guess was about 850,000-900,000. Got to within 500 ft of the main stage, it was quite an event.