Cerritos park
© CBS LAThe scene at Cerritos Regional Park.
Authorities say three people were sent to the hospital after a large swarm of bees attacked families picnicking in Cerritos.

Fire officials say the incident happened near the tennis court at Cerritos Regional Park on Bloomfield Avenue on Saturday afternoon, where a family reunion was taking place.

"It was thousands. It was thousands, like this whole grass area was covered with bees," said one park visitor. "They were chasing hundreds of people."

Witnesses say at least 20 people were stung, and cellphone video captured the aftermath.

"The more women or men that had long hair, they were just attacking them," said another park-goer. "It was just so crazy."

The bees reportedly emerged from inside an underground utility box on the grassy field.