According to the Los Angeles County Sheriff's Department, around 10:30 a.m., the arboretum reported seeing the bears at the facility.
Arboretum officials said on any given day, there could be about 1,000 visitors to the site.
"Around 11 a.m. this morning, the county sheriff thought it would be wise to ask our visitors to leave the arboretum," said CEO of the arboretum Richard Schulhof. "The bears were asleep on the trees, but we just thought, on the side of caution, that was the right thing to do. So, we asked our visitors to leave the premises."
Everyone was evacuated safely. Workers remained on site to help with security measures such as closing gates.
According to arboretum officials, the bears were a mother bear and her two cubs.
Residents in the area said bear sightings are common in the neighborhoods closer to the mountains and not past the 210 Freeway.
California Fish and Game officials were also on-site to safely remove and relocate the bears.
Arboretum officials said this is most likely the first time bears were seen on the premises.
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