Forty minutes before Public Enemy take to the stage, Chuck D sits Buddha-like in the corner of the dressing room. The door bobs open and shut, open and shut. Members of the entourage pop their heads round briefly, TV crews nudge the door hopefully, the PR darts in and reminds us that we have 15 minutes. Outside in the corridor, their Uzi-wielding dance troupe, Security of the First World, are rehearsing their dance steps. Minutes earlier, in the same spot, Flavor Flav, the band's enfant terrible, had been spitting grapes on to the floor in an elaborate photo-shoot performance. Now the dancers' boots pound a heavy one-two against the grape mush.

Public Enemy have been quite simply the most revolutionary act in rap history. Behind Chuck D, the line-up has, intermittently, included Professor Griff, Flavor Flav and DJ Terminator X. In 1987, they propelled themselves to notoriety with the single Public Enemy No 1 and the following year produced the seminal album It Takes a Nation of Millions To Hold Us Back. The music was bullish, chaotic, their voice strongly, militantly pro-black and controversially aligned with the teachings of Louis Farrakhan, leader of the Nation of Islam. Chuck D rapped about the problems blighting the black community, and memorably described hip-hop as "the black CNN". In Black Steel in the Hour of Chaos, for example, he questioned whether black soldiers ought to fight in the US army. More recently, he has attacked the American president in Son of a Bush: "I ain't callin' for no assassination/ I'm just sayin' who voted for this asshole of the nation."

Chuck D has long been cited as rap's most articulate spokesperson, appearing regularly as a guest speaker on the Fox network and touring some 50 US colleges a year to speak on the subjects of "rap, race, reality and technology". "Those," he explains, "are issues and topics that swirl around the collegiate mind." Chuck's own musical career was born on college radio at Adelphi University on Long Island, New York.

This week sees the release of We Are Gathered Here, the debut album from one of Chuck's many side-projects - the Fine Arts Militia. The press release describes them winningly as a "rock/funk/jazz/hip-hop quintet mixed with lyrical views on rap, race, technology and a post 9-11 world." We Are Gathered Here is effectively a series of college lectures set to music. Fine Arts Militia debuted on January 31, at Not in Our Name, a conscientious objectors' concert in Berkeley, California, clad in black suits, white shirts, and skinny liquorice-strip ties.

Today, Chuck is in more predictable attire: black jogging bottoms and black T-shirt, a little knitted black hat pulled snugly over his head. He is trying to explain Public Enemy's continuing role in hip-hop. "We are pretty much the ambassadors of the art form," he says, slowly. "We're the ones who had to secure the people inside at the party when somebody had to be at the door." He has one of those bear-like voices, a low, humming, almost-growl. "Once upon a time," he continues, "there was no door to watch. Now that whole wall's been gutted out, so you pretty much have to secure whatever is left in rap music and hip-hop. Anybody can come in now. But you can still navigate people to understand that it's an art form that inherited a lot of the legacies of black music."

He speaks like this a lot - in long, sticky sentences that are difficult to unravel, all the while levelling an unflickering gaze upon you. "In the past, people maybe rapped and they did the music for the masses of the people," he says, in the sort of world-weary tone only a distinguished rap ambassador could muster. "Rap was anti-elitist, and anti-establishment, it wasn't turning its nose up at the masses. Now it's a bit troubling to see that where rap was rap for the people, now it's become the elite, speaking against their people. Rap is rap for the companies, rap for the corporations. We," and here I think he means Public Enemy, "are trying to put some of the balance and diversity back, to restore some of that."

He talks a lot about "the masses". "The masses of the people are forgotten," he argues, "because the masses of the people are ruled by the few. Whether that be the UK, America, Iraq ... all of them. All of the governments of the world could do a better job of answering to the masses, as opposed to treating us like ..." and here he starts to laugh, " ... the masses."

Chuck's latest way of communicating with "the masses" is via the internet, which he describes as the most exciting thing in hip-hop today. Public Enemy's 1999 album, There's a Poison Going On, was one of the first to be released on MP3 format, with the subsequent tour webcast live on the internet. "To have to submit art to some intermediary, to me was not purposeful to the growth of the art," he explains. "I was having to deliver it to this intermediary in order for it to get to the public. So for me, the internet was a saving grace. Whether it's for the delivery of video, audio, ideas, and loosely building something around those things."

Indeed, Public Enemy's latest album, Revolverlution, includes four tracks reworked by fans on "internet first" record company slamjamz.com. The album's artwork and sleevenotes were also created by fans that Chuck D encountered on the site's message boards.

His next project is a syndicated internet radio programme, a combination of music and talk, in part a backlash against corporate-owned radio in the US, which has "kind of reduced the streets, or street cred, to be something that is automatically indigenous to black culture. Where once upon a time we were telling kids to keep off the streets, now the street is invited into people's homes because of media such as television and radio." He stops and stares at me. "Do you think that companies concoct a culture inside their boardrooms and create the cultures on the streets?" he asks. It is not a rhetorical question. I nod my head. Seemingly, this was the right answer, because he continues.

"So a lot of these situations are almost like sewage companies at the top of the river, who also have a bottled water company at the bottom of the river. It's like you can't pour sewage into the river and expect to have uncontaminated water! So when it comes down to, say, the hip-hop summit, many of these cats were the same companies that created the situation."

The hip-hop summit in 2001 was a conference called to address the increasing problems of hip-hop-related violence in America. We discuss Britain's similarly increasing level of gang-related crime, and its alleged relation to hip-hop. "When you have increased Americanisation, suddenly all the other countries around begin to lose their own identity," he says. "And that [American] influence can come from the media. And entertainment rides media quite well. How it comes through that strainer, that interpretation can be very influential to the people that don't have a sense of themselves."

As roving ambassadors, Chuck D evidently feels it is Public Enemy's responsibility to restore that sense of self. Later that night, as their set pounds to a close, he salutes an audience that is a predominantly white, druggy crowd. "Think for yourselves London," he bellows. "Think. For. Your. Selves."