Three people are dead and at least 27 others are injured after a suspected people-smuggling boat broke apart off the California coastline.

As search and rescue teams arrived on scene, dozens of people were spotted in the water after a 12-metre “severely crowded” boat was smashed to pieces by the surf off Carbillo National Monument at California’s Point Lomo about 10am on Sunday local time.

San Diego Lifeguard Chief James Gartland told KARE news: “This was a mass rescue operation that turned into a mass casualty event”.

“It’s a tragedy.

“It’s a tragic event here in San Diego. Probably one of the bigger vessel accidents, bigger calls that we’ve seen here certainly in my 26 years and certainly in Lt. Romero’s 28 years – this is probably the worst tragedy,” he said.

San Diego Fire-Resce lieutenant Rick Romero said a large emergency response was initiated, including calling in the US coast guard and state and local agencies, after the magnitude of the incident became clear.

San Diego lifeguards and rescue agencies spotted up to 30 people in the water, initially performing seven water rescues and one cliff rescue. Many required life-saving efforts on the beach.

“Our goal was just to rescue everyone we can from the water and along the beach and get them safely transported to the hospital as quickly as we can,” Lieutenant Romero said.

Tweet from @SDFD

There had been about 30 people on board the boat in severely crowded conditions without adequate safety equipment, authorities said.

Border Patrol agent Jeff Stephenson said it appeared to be a smuggling vessel but it was not yet clear as to the nationality of the people on board.

“Every indication from our perspective is that this was a smuggling vessel used to smuggle migrants into the United States illegally,” he said.

He described the boat as “severely overcrowded.”

“[Smugglers] don’t care about the people they’re exploiting,” Agent Stephenson said.

He added that the captain was in custody and speaking with investigators.

Up to 100 rescuers treated multiple patients on clifftops during the rescue effort. Photo: AAP

Authorities said they had seen a marked increase in maritime smuggling in recent years.

The vessel involved in Sunday’s incident was larger and had more people than most smuggling boats, according to the Border Patrol.

Cabrillo National Monument is a park with trails, tide pools and exhibits on California history, operated by the US National Park Service.

It is named after Juan Rodriguez Cabrillo, who stepped ashore in 1542 as the first European to arrive on what is now the West Coast of the United States.

-with agencies