Harrison and three campus police colleagues are suspended in a cage from a cherry picker, and are talking to the tree-sitters, who are perched on a redwood tree about 90 feet above ground.
Mogulof said that if the tree-sitters don't surrender voluntarily, the university "will continue with its preparation to bring this situation to a safe conclusion."
He declined to be more specific.
One of the tree-sitters had climbed to a perch on the very top of the redwood tree and at one point gave a two-fisted victory solute to the cheers of supporters below.
On Monday, Mogulof said the university planned to move quickly to remove the tree-sitters so that it can move forward with its plan to build a $125 million sports training center next to the football stadium.
About 30 supporters of the tree-sitters have gathered on Piedmont Avenue to watch the situation unfold.
Many of the protesters are banging on drums as a show of support and others are yelling at campus police officers.
Police detained one man who was attempting to block traffic on Piedmont Avenue and an officer aggressively pushed back another protester who was in the street.
The university began tearing down trees at the construction site on Friday morning, the day after the state Court of Appeal rejected a request by opponents of the project to maintain an injunction that had halted work for 21 months.