Counsellors said they witnessed many more than the four complaints police have received, some in full public view in the "mosh pit", the dancing area in front of the stage where festival-goers "surf the crowd".
David Schneider told the Washington Post he saw women lifted into the air and then pulled down into the pit: "I saw someone push this girl into the mosh pit ... Then a couple of the guys started taking her clothes off ... They pulled her pants down and they were violating her, and they were passing her back and forth. There were five guys raping this girl."
He said he saw at least five similar assaults. No one, including guards near by, tried to intervene, probably because the crowds were so dense. Mr Schneider said: "There wasn't anything I could do. They're big brawny people and it seemed ... most of the crowd around them were cheering them on."
A Red Cross worker, Sandy Lattimore, said she had treated victims of sexual assaults that had taken place in the pit.
A woman whose daughter said she was pulled into a tent and raped posted a notice on the Internet offering a $10,000 (pounds 6,250) reward for help in finding the perpetrator. "Everything was great at Woodstock until the early hours of Monday morning, when my daughter got separated from her three girlfriends. A clean-cut college- looking guy with dirty blond hair pulled my daughter into a tent and raped her. There were people around and must have heard her screams and the struggle going on inside. We need help; if anyone saw or heard anything please e-mail me," the notice read.
Organisers of the event, held at a disused military base 200 miles from the farm where the original Woodstock was held in 1969, would not confirm the assaults and asked whether it would be possible to assault someone in such a crowded space as the "pit". "What about 199,000 kids that came and had a great weekend?" asked the promoter, Peter Scher. The co-promoter, Michael Lang, said: "I don't think it's conceivable", when asked if it was possible to assault someone sexually in such circumstances.
But Rosemary Vennero, from a centre that counselled four alleged victims, said the festival was "a perfect breeding- ground for sexual assault" because of the combination of heat, alcohol, drugs and crowded conditions.
Sexual assaults in mosh pits at concerts are not unknown, said some crowd- management experts. Paul Wertheimer said security guards often assumed that if women went into a mosh pit and body-surfed, they were "somehow inviting that kind of thing".