City spokesman John Tulloch called the mudslide, which was reported shortly before 7 a.m., "pretty major."
Fueled by heavy overnight rains, the slide spread from the canyon's north side across the roadway, knocking down trees onto power lines, Tulloch said.
PG&E and Piedmont Public Works Department crews were responding to the area, but Tulloch said they will not be able to determine a course of action until PG&E can de-energize the power lines and remove the hazard posed by the downed trees.
Because public works crews have to keep a safe distance, he said, they have not been able to determine if the mudslide affected a home uphill from the slide.
Officers shut down Moraga Avenue between Red Rock Road, also known as Coaches Field, and Estates Drive after receiving word of the mudslide, Detective George Phifer said.
Police have also blocked off traffic coming from Maxwelton Road to prevent cars from reaching that portion of Moraga Avenue, Phifer said.
Because the city has yet to assess the stability of the hillside, Tulloch said it was unknown when the roadway will reopen.
"When it's safe, we'll get the road back open," Tulloch said.