Students shared photos of the defacement -- which comes as several schools have faced recent racial turbulence on campus -- on social media Thursday morning.
While the identity of the perpetrator is unknown, Leland Shelton, president of the Harvard Black Law Students Association, said he believed the crime was committed by a "coward."
"[This person] wasn’t brave enough to say these types of things in a classroom discussion, to raise [his or her] racist ideas. [He or she] crept in the middle of the night and did this."
Harvard police would not comment beyond confirming that a hate crime investigation was underway, according to spokesman Steven Catalano.
More than 300 concerned students, faculty and staff met Thursday at noon to discuss the incident after students pressured the administration, Shelton said in a statement.
Prior to the defacement, HLS students participated in a "Day of Activism" with neighboring universities to stand in solidarity with black college activists nationwide.
Ronald Sullivan, clinical professor of law at HLS, had his portrait defaced. The director of the Criminal Justice Institute said the incident exposed what he said were underlying issues at Harvard.
"It is some form of erasure and I can really empathize with the students," he said. "I knew I'd be fine but they don't feel that their voices are at all valued. It was symbolic: we don’t value you, we don't value people that look like you."
Hours after the defacement, the black tape was replaced with Post-It notes featuring positive messages about the professors of color.
Student Body President Kyle Strickland said student organizations will hold separate meetings in the next few weeks to address the issue.
"We need people literally from all walks of life to enter this discussion and we need folks out there disagreeing," Strickland said. "This is going to be a heated debate, but it’s only going to be inclusive if we make sure every voice is included. This is not going to be something that is swept under the rug."