Doug Rider, Pacifica's chief building official, said the evacuation order was issued after part of the cliff, saturated by recent storms, broke off around 5:30 a.m.
An initial evacuation deadline of 5 p.m. was moved up as the cliff continued to crumble. Moving trucks filled the street as about 20 frantic residents packed up what belongings they could.
"It's been insane. I can't even think," said Amanda O'Connell, 24, a swim teacher in this town of 38,000 about 10 miles south of San Francisco.
O'Connell said residents were warned earlier in the week about the deteriorating bluff. She and her boyfriend decided to move some of their larger possessions out early, just in case, she said.
"This whole week has been the most stressful week of my life," she said.
The other two buildings remained stable late Thursday night.
Rider earlier credited their stability to a wave-deflecting barrier of boulders at the bottom of the bluff. The barrier was erected last spring but did not protect the entire beach. The cliff began crumbling before the barrier could be completed.
The building's owners have received an emergency permit from the California Coastal Commission to put a barrier in place, Rider said. Work was scheduled to begin Friday with a crane lifting boulders over the building down to the beach, he said.
The erosion of the bluff's sandy soils is a natural occurrence exacerbated by the region's wet winters. Officials had been monitoring the buildings since recent heavy rains. If the barrier halts the erosion, residents could return to the building eventually, Rider said.
Rick Jenkins, 62, lives in the building next to the evacuated apartments. He said the sound of the cliff breaking apart Thursday morning was like thunder or an earthquake.
Much of the cliff in front of his own home had fallen away about 18 months ago before the boulders were brought in, he said. Gesturing toward the wide blue expanse of ocean that greets him every morning, he said the risk was worth it.
"You either move or you don't," he said. "I guess when they say it's time to go, you go."