A video of a 30-year-old Barack Obama making a speech at Harvard Law School has finally appeared on YouTube, following claims by conservatives that it would portray him as a "radical".

As president of the Harvard Law Review in 1991, Obama defended Prof Derrick Bell during a protest arguing for racial diversity on the faculty of law.

Bell, a pioneer of "critical race theory", was popular at the time for teaching about race and power. After Harvard denied tenure to Regina Austin, a black woman professor, Bell took an unscheduled leave of absence and said he would stay away from the job "until a woman of colour is offered and [has] accepted a tenured position on this faculty". He also went on hunger strike to emphasise the point.

Only three of the law school's professors were black and only five were women.

In the video, Obama engages in a passionate defence of Bell, "the Rosa Parks of legal education", as he described the professor in another speech.

Rosa Parks was an African-American civil rights activist who became an icon for the fight against racial segregation in the US when she refused to obey a bus driver's order to give up her seat to make room for a white passenger.

Obama praises Bell for his accomplishments with the scholarship programme at Harvard. "He hasn't done it only by good work and easy charm, although he has both and in ample measure," he says in a passage of the one-minute footage.

The video was discovered by Andre Breitbart, a conservative blogger. He told right-wing activists in Washington DC: "I've got videos from his college days to show you why racial division and class warfare are central to how 'hope and change' was sold in 2008."

He claimed the clip would show how Obama had been plotting to take power for more than 20 years. After Breitbart's death recently, bloggers speculated that the video could affect the course of this year's election.

Instead, the footage shows nothing more than a self-confident, conciliatory Obama, already at ease with holding a crowd's attention.

"Barack was always supportive and sympathetic to our campaign for faculty diversity. He spoke about it at one of our rallies. But he was not actively involved in the protest movement," said campus campaigner Keith Boykin.

"And even if he agreed with the cause of the movement, he didn't need to be involved in the more radical protests we launched because our tactics were controversial on campus," Boykin added.