HYANNIS — Six people were injured, three of them seriously, when a high-speed ferry crashed into a jetty at the entrance to Hyannis Harbor Friday night, authorities said.

The Iyanough ferry was carrying 48 passengers and nine crew members from Nantucket to Hyannis when it struck the jetty and grounded on the rocks around 10 p.m., the US Coast Guard said.

One seriously injured passenger was airlifted off the stranded vessel, and then flown by helicopter to a hospital for treatment of undisclosed injuries, said Hyannis Fire Capt. Thomas Kenney.

Other injured passengers were treated onboard the ferry by paramedics and rescue crews that were able to reach the ferry, Kenney said.

But several other passengers had to be airlifted off the vessel when sea and air conditions made it too difficult for crews to reach the Iyanough, he added.

At the time of the crash, three-foot seas were reported in Nantucket Sound, with winds gusting to 15 to 30 miles per hour, according to the National Weather Service in Taunton.

Multiple agencies from Cape Cod raced to the scene after the vessel sounded a mayday call just after 9:30 p.m., according to Kenney.

“It was a mayday, radio mayday, that was intercepted by our fire alarm office,’’ he said.

Kenney said he immediately notified the Coast Guard. Hyannis fire and police responded by ground and water, he added.

The rescue effort continued into early Saturday morning as the Coast Guard attempted to retrieve passengers. Helicopters cirled the ferry and nearby rescue vehicles, parked on what otherwise would have been a quiet street by the Hyannis Port Yacht Club.

Police blocked off the pier to the public, but onlookers gathered around the scene to get a glimpse of the ferry that apparently veered off course.

The jetty, a giant pile of rocks extending several feet into the harbor, is near the Kennedy compound in Hyannisport, Kenney said.

“If you were standing on the . . . Kennedys’ front lawn, it would be right in front of you,’’ Kenney said. “the boat drove up on that pile of rocks.’’

Kenney, a 36-year veteran of the Hyannis Fire Department, said he could not remember a time when a ferry had struck the massive jetty.

“I’ve had smaller boats hit that, maybe 15- to 25-foot-range boats, but this is the largest boat that’s ever hit it in my career,’’ he said.

The Iyanough is operated by the Steamship Authority, which runs ferries to Nantucket and Martha’s Vineyard. Attempts to reach the company Friday night were not successful.

On its website, the Steamship Authority describes the Iyanough as a 154-foot, high-speed ferry that travels from Nantucket to Hyannis in an hour. It carries up to 400 people.

The Coast Guard dispatched boats from stations in Woods Hole, Chatham, and Brant Point on Nantucket, and a crew from Air Station Cape Cod, the agency said.

Rescue crews were scheduled to work into Saturday morning to “oversee the safe transfer of all passengers and ferry crew,’’ the statement said.

The cause of the grounding is under investigation, according to the Coast Guard.

Jacob Geanous can be reached at jacob.geanous@globe.com. Rowan Walrath can be reached at rowan.walrath@globe.com