Exactly 40 years ago, half a million people gathered for the ultimate event of the counter-culture movement. What they remember is three days of peace, love and most importantly, music.

Woodstock was the brainchild of Artie Kornfeld, a music executive, and Michael Lang, a concert promoter. The pair had responded to an advert in the New York Times placed by John Roberts, the heir to a pharmaceuticals fortune and Joel Rosenman which read: "Young men with unlimited capital looking for interesting, legitimate investment opportunities and business propositions."

In April 1969, the four men met up and formed Woodstock Ventures.

"I was 24 years old, and the vice-president of Capitol Records," Kornfeld says. "Michael Lang and I were great friends. One morning, about 3am, we were playing pool and he was teasing me, saying I'd been writing and producing so much that I didn't go to clubs any more and see bands. We said wouldn't it be great if all the bands I never saw played a free show to which we'd invite all our friends.

"I knew Jimi [Hendrix], so, once Jimi said yes, you couldn't keep the acts away. The message was all about freedom of choice, love your brother, everyone treat each other better."

Michael Lang says, "We always wanted to have it as a counter-culture event: it was important to me to have it about politics, interests in ecology and human rights. We deliberately wanted to create a space where everyone was welcome. If you didn't have money to eat, there were kitchens for you; if you didn't have a tent, we had a free camp site; and those who couldn't afford tickets, we'd get them in anyway," Lang says.

"Once you got within 100 miles of the place, it was like a magnet," Kornfeld says. "There was a spirit of brotherhood. People came together as one, and everyone was sharing food. We had to close the roads from New York because three million people were on their way. People were just leaving their cars on the freeway. It was anything goes."

Chip Monck (described by Martin Scorsese as "the pioneer genius of rock-concert lighting", was the lighting director of the festival and last-minute master of ceremonies). "On the Friday morning, Michael clapped me on the back on and said, 'We haven't hired an MC - you're it,'" Monck says.

"The first thing I did was ask 500,000-plus people to pick up all their belongings - sleeping bags and everything - and take 10 steps back. Amazingly, they did. Then we put up two metal sticks and a clothes line as the barrier in front of the stage. That flimsy little barrier was never breached the whole three days. There was such a relaxed vibe. I'm sure not a punch was thrown throughout the festival.

"When the rain came on the Saturday, boy, did it hit, but it only drew people closer. Everyone looked the same - like drowned rats. It was a major unifying factor. Many of the artists weren't in great form. They had taken too much before coming to the festival, so we had to look after a lot of them," Monck says.

Arlo Guthrie, the folk singer, famous for his protest songs, performed at Woodstock on the first night - much to his surprise. "The first sign of a this-is-different scenario was when we couldn't actually get close to the site," Guthrie says.

"All the roads were jammed, so we were diverted to a motel in the forest in the middle of nowhere, where helicopters were ferrying the acts to and from the stage. I was 19, and I'd never been in a helicopter in my life, so flying in over those crowds was something else. No matter how high we were, there were people as far as the eye could see. I was under the impression that I was to perform on the second day, so I was dropped off backstage, and started wandering around, saying hi to friends. I had a real case of the munchies in the afternoon, so I headed out into the fields of mud to find something to eat, but there was nothing to eat or drink anywhere."

Barry Levine was the stills photographer for the Oscar-winning Woodstock film. During his six days at the festival, the only sleep he got was a 45-minute nap on the piano cover on stage during the performance by Blood, Sweat & Tears. "The vibe at Woodstock was one of people understanding that there is a little heaven in every disaster. There was not enough food, water, toilets. It was uncomfortably hot and wet, but nobody complained. Everyone was determined to show the world that, given the opportunity, there was a different way of doing things from the traditional 2.3 children, buttoned-down-shirt way of life, and we knew the whole world was watching.

"There were plenty of cops around, but they didn't bother anyone. Folks were skinny-dipping and walking around nude. At the time, having long hair in the US was enough to get you beaten up, and espousing peace and love drew accusations of being a communist. But, when half a million people came together to share that ethos, it made you feel that you weren't alone, that we were a movement, that, no matter what your beliefs were, we were united on some very basic issues.

The success of the festival has arguably fuelled every outdoor rock event since. It sparked The Concert for Bangladesh, the star-studded charity show organised by George Harrison in 1971, which itself was an inspiration for both Live Aid (1985) and Live 8 (2005). Woodstock was also the precursor of Glastonbury, now in its 39th year of raising funds for environmental concerns.

Says Kornfeld: "Time magazine called it the greatest peaceful man-made event in history. The impact was like the war of the worlds, a time of love and hate. I think it was also the start of the end of the war. Forty years on, that still resonates and people can still see the significance of it. I think the anniversary is focusing people on the problems we still have today. But you can't do another Woodstock because you can't repeat a miraculous work of art."

Lang agrees. "I think what Woodstock represented, and what it still represents today, is hope, which is in such short supply. Woodstock gave the hope that things could be different. A lot of people describe the inauguration of Barack Obama as a Woodstock moment. We've had eight dark years, but now there's hope."

Says Lang: "Woodstock sowed the seeds for the green movement - nobody talked about organic gardening before Woodstock or had seen granola or long-grained rice, and all our kitchens on site used organic, locally-grown vegetables.

"You can't plan magic like Woodstock. Glastonbury, Live Aid, all the others - they are all wonderful music events, but they haven't had the impact on social behaviour that Woodstock did. People who were there, and even people who watched it on TV, came away with a different take on how we can live with each other."