Body-sized cardboard effigies of lynching victims were seen hanging by nooses at the University of California, Berkeley, amid nationwide protests over the killings of black men by police officers.

University police responded to morning reports of two cardboard cut-outs in public campus areas before a midday demonstration and march assembled on the campus in the Oakland area, spokeswoman Claire Holmes said. A third figure was also hung before all were taken down.

"It has been unclear to us whether this was racially motivated or part of the protests across the country against police violence," Holmes said, adding the images were disturbing and the school would investigate. There were no suspects.

Two of the effigies, a black woman and black man, could be seen in images on Twitter with "I can't breathe" written across them, referring Eric Garner's words to New York police as they subdued him, and which have become a rallying cry in demonstrations nationwide. The woman bore the name of Laura Nelson, who was lynched in Oklahoma in 1911.