According to the lawsuit, Fresno State Students for Life planned to write messages in chalk outside the university's library. After getting approval from the university, they set a date: May 2.
That morning, the group wrote their messages; examples ranged from "You CAN be pregnant & successful" and "Unborn lives matter" to "Women need love, NOT abortion."
Shortly before finishing, the lawsuit claims, William Thatcher, an assistant professor of public health, approached the group and said they had to keep their messages to the university's free speech area.
According to Bernadette Tasy, one of the organizers with the group and a plaintiff in the lawsuit, Thatcher then said he would be back to remove the messages. Tasy says he returned with a group of students and they attempted to remove the group's messages.
Video provided to CNN by the group's lawyers shows Thatcher starting to erase the messages with his foot.
"You had permission to put it down; I have permission to get rid of it," he says in the video.
Tasy claims the students also took some of their chalk and wrote "pro-abortion messages" on the same sidewalk. The messages included "My body, my choice" and "Your body, your choice. I (heart) you."
The lawsuit alleges that Thatcher got students from his 8 a.m. class to help remove the anti-abortion messages.
CNN has been unable to obtain comment from Thatcher despite repeated requests.
In a statement to CNN, Joseph Castro, president of California State University, Fresno, said the school's policy on free speech is clear.
"Free speech on campus is not limited to a 'free speech zone' or any other narrowly defined area," he said. "Those disagreeing with the students' message have a right to their own speech, but they do not have the right to erase or stifle someone else's speech under the guise of their own right to free speech."
After the incident, the students contacted lawyers at Alliance Defending Freedom
, a non-profit Christian legal group, which took up the case.
"The crux of this case is a very simple message. Public university professors should be encouraging free speech," said lead attorney, Travis Barham, one of the lawyers from the group, "not erasing it from existence. You will be held accountable."