A University of Oregon law professor was placed on administrative leave Thursday after wearing a costume that included blackface to a Halloween party attended by faculty and students.

“We condemn this action unequivocally as anathema to the University of Oregon’s cherished values of racial diversity and inclusion,” the university’s president, Michael Schill, said in a written statement. “The use of blackface, even in jest at a Halloween party, is patently offensive and reinforces historically racist stereotypes. It was a stupid act and is in no way defensible.”

Many college officials warned students this fall, at a time when racial tensions are high, to think about their Halloween costumes to avoid offending others with cultural stereotypes.

The idea that a faculty member at the School of Law would wear blackface shocked students and others.

Before Halloween, the Black Student Union planned a meeting about cultural appropriation, explaining to students how costumes that play off cultural stereotypes can be offensive to other. “We were expecting this to come from students,” said Natashia Greene, a sophomore. “Not a grown woman. For a law professor, an adult, a grown woman — for us to worry about a professor — that is where the pain comes from.  … That makes it hurt way more. She teaches us. She’s in charge of our grades. She’s in charge of our education.”

Samantha Berguin, a junior who is co-director of the Black Student Union, said she was frustrated “that a person employed by the university would think that was acceptable, and there was no repercussion … that hurt me,” and made her worry about other black students on campus.

In a similar incident last year, the president of the University of Louisville apologized for wearing a stereotypical Mexican costume at a Halloween party.

The University of Oregon professor has apologized, Schill wrote, and the Office of Affirmative Action and Equal Opportunity is determining whether the costume is a violation of university policy.

That led some to question whether the professor’s free speech was being hindered at a public university.

But some law professors called on their colleague to resign if the allegations are true. “It doesn’t matter what your intentions were. It doesn’t matter if it was protected by the First Amendment,” the letter signed by 23 faculty members said. “Blackface is patently offensive. It is overtly racist. It is wildly inappropriate. It reflects a profound lack of judgment. There is no excuse.

“We are angry that you would alienate our students, staff, and faculty of color. We are angry that you would destroy what others have worked hard to build.

“Your actions implicate all of us and our community.

“If you care about our students, you will resign. If you care about our ability to educate future lawyers, you will resign. If you care about our alumni, you will resign.”

The university declined to confirm the name of the professor, saying it is a personnel matter.

Schill, who is also a faculty member in the School of Law, said the incident shows the need for more training and dialogue on racial issues, and cited a workshop and lecture series. He said implicit bias training would be required for all faculty-search-committee members, and “new training on micro-aggressions” will be offered this winter.

“Bigotry and racism have no place in our society or at the UO. Providing a welcoming, diverse, and inclusive environment for all is one of the university’s top priorities. We have been working for more than a year with our students to further these objectives,” Schill wrote in the letter, which was also signed by other top leaders at the university.

Earlier this fall, the university announced that a dorm that honored a professor who had been a leader in the Ku Klux Klan would be renamed, responding to student protesters’ anger about the name.

“Last year black students organized and created a list of demands,” said Ashely Campbell, a junior who is co-director of the Black Student Union. “We’re trying to make a change in campus climate, trying to make it more comfortable for black students. For the teacher to do this just showed she didn’t care about the efforts we’re making. To have that costume is kind of like a slap in our faces.”

More than 600 people had signed an online petition Thursday demanding the professor’s resignation, writing, “We alumni, faculty, staff, current students and greater community members are deeply offended and outraged. . . .”

Greene said over the past year, “I feel there was progress that was made — not even just on this campus. There’s a lot of issues going on in society with black people now. We’re trying hard. We’re trying really hard. For the professor to take the step she took, it did affect us badly. It was a step backward.” But the black organizations on campus are strong, she said. “We are united. We can put this step backward into a step forward.”