The top U.S. court is considering whether universities can use race as a factor in assessing students for admission.
The case pits the University of Michigan's law school against white students who were rejected. But the U.S. Supreme Court ruling, expected by July, could rewrite affirmative action laws across the country.
The administration of President George Bush has backed the rejected students, while up to 7,000 people, most in favour of affirmative action, demonstrated outside the court Tuesday.
The law school, one of the top 10 in the U.S., tries to ensure that minorities represent 10 to 12 per cent of each class.
That's unfair, said Jennifer Gratz, a white student who was rejected and a plaintiff in the case. She said with her grades, she would have been admitted if she were black, Hispanic or aboriginal.
"I was treated unfairly because of my skin colour," she said.
A lawyer for the students argued that promoting diversity does not justify giving preferential treatment to minorities. He was backed by Bush's Solicitor General Theodore Olson, who said Michigan is using stereotypes to break down stereotypes.
The university's lawyer said diversity enriches all students.
Outside the court, minority students said affirmative action had given them a chance they otherwise would not have had.