The three bears were reported at about 8 a.m. on the 1400 block of Edgecliff Lane, according to the L.A. County Sheriff's Dept. State Fish and Game Dept. officials were called to the scene to assess the danger to the public.
A crowd formed near the tree to observe the bears.
A Fish and Game Dept. spokesman said they would likely leave the bears alone in the tree, and that they would probably remain in the tree until after dark when the neighborhood is quieter. The bears will probably come down on their own then, according to the spokesman.
"The most important thing people can do is stay away," said spokesman Andrew Hughan. "All it's doing is scaring them."
Shortly after 2 p.m., the bears climbed down the tree next to a house and wandered through two back yards. The bears then climbed another tree.
Fish and Game officials say it appears the bears woke up recently from winter hibernation.
"We had a warm spell I believe about a week ago, so that really brought them up," said Fish and Game Warden J.C. Healy. "They're starting to move down. They're looking for water, they're hungry."
Bear sightings are not uncommon in the Altadena area so close to Angeles National Forest.
"It's right above a ravine or a canyon, and that canyon will go south and basically hook up to another canyon, which hooks up to the larger canyon, which will take her back up north, which is what we want," said Healy. "So this is actually working out quite well for us and for mama bear and the two cubs."
Tranquilizing the bears was decided against because they don't like introducing drugs to a bear that's nursing.
The bears climbed down from their tree about 6 p.m. and made their way into a stand of trees.