From Anarchy in Action
The Woodstock Music & Art Fair, was a music festival held on Max Yasgur's farm in Bethel, New York from August 15 to 18, 1969. Some 40,000 attendees in effect lived for 3 days in a free city. Encyclopedia Britannica reports that security forces were "virtually nonexistent". There were 2 recorded fatalities, one from an apparent heroin overdose and one when a tractor driver accidentally ran over someone sleeping in a hayfield. 
Up Against the Wall Motherfuckers, an anarchist collective based in New York City, cut the fences at Woodstock, allowing people to enter the festival without paying.
Colin Ward, Anarchy in Action:
[Spontaneous order] could be seen in spite of commercial exploitation in the pop festivals of the late 1960s, in a way which is not apparent to the reader of newspaper headlines. From 'A cross-section of informed opinion' in an appendix to a report to the government, a local authority representative mentions 'an atmosphere of peace and contentment which seems to be dominant amongst the participants' and a church representative mentions 'a general atmosphere of considerable relaxation, friendliness and a great willingness to share'. The same kind of comments were made about the instant city of the Woodstock Festival in the United States: 'Woodstock, if permanent, would have become one of America's major cities in size alone, and certainly a unique one in the principles by which its citizens conducted themselves.'
- Horizontal Power Hour, "Ben Morea and Black Mask", 10 April 2012, https://horizontalpowerhour.wordpress.com/2012/04/17/episode-39-ben-morea-and-black-mask/.
- Fifty Million Volunteers, Report on the Role of Voluntary Organisations and Youth in the Environment (London, 1972).