JOHANNESBURG, South Africa (CNN) -- Black students on a university campus in central South Africa resumed their protests Tuesday against a racist video that showed white students tricking black workers into tasting stew laced with urine.

Students protest against a racist video on the campus of Free State University in Bloemfontein, South Africa.
Protesters barged into classes at the University of the Free State in Bloemfontein, demanding teachers suspend lectures. One lecturer who refused got into a confrontation with students.
Authorities arrested 28 protesting students.
The students want classes boycotted until Friday, when university officials are expected to decide whether they will close the residence hall where the video was made.
Watch as video sparks protests »
The video, which surfaced Tuesday but was made in September, was a reaction to the university's efforts to integrate its residences, the university said in a written statement. It has sparked widespread outrage and condemnation.
The university, located about 400 kilometers (245 miles) southwest of Johannesburg. immediately suspended two of the students involved; the other two completed their studies last year. A criminal investigation is under way.
In the video, white male students at Reitz Residence are seen encouraging at least five black housekeepers to participate in what the students call the "Reitz Fear Factor," an apparent reference to a television show in which contestants eat live worms or compete in other feats.
In one scene, a student mixes what looks like a beef stew in a plastic bowl and adds garlic and other items. Then he tells the camera he will add the "special ingredient."
The student then urinates into the mixture, which he later stirs up and puts in a microwave. Other students can be heard laughing on the tape.
The next scene shows a different student urging at least three housekeepers to drink cups full of the stew, saying, "This is our dorm's 'Fear Factor.' We want to see who has the best 'Fear Factor.'"
On the video, the student does not tell the women that there is urine in the mixture.

The women, on their knees, spit the stew into buckets after tasting it. Some appeared to vomit, but the women also laughed during the incident, as the student urged them on.
At the end of the video, a message appears on the screen in Afrikaans saying, "That, at the end of the day, is what we think of integration." E-mail to a friend ![]()
CNN correspondent Nkepile Mabuse contributed to this report.
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