A portion of the Francis Scott Key Bridge in Baltimore collapsed after a large boat collided with it early on Tuesday morning, causing multiple vehicles to fall into the water.

Authorities and rescue crews say they are working to rescue at least seven people.

The vessel appears to have hit one of the supports of the Francis Scott Key Bridge, causing the roadway to break apart in several places and plunge into the water.

The ship caught fire and thick, black smoke billowed out of it.

“This is a dire emergency,” Kevin Cartwright, director of communications for the Baltimore Fire Department, told The Associated Press.

“Our focus right now is trying to rescue and recover these people.”

He added that some cargo appeared to be dangling from the bridge.

Emergency responders were searching for at least seven people believed to be in the water, Mr Cartwright said, though he said it’s too early to know how many people were affected.

He called the collapse a “developing mass casualty event”.

He said agencies received 911 calls around 1.30am reporting a vessel travelling outbound from Baltimore that had struck a column on the bridge, causing it to collapse.

Several vehicles were on the bridge at the time, including one the size of a tractor-trailer truck.

From a vantage point near the entrance to the bridge, jagged remnants of its steel frame were visible protruding from the water, with the on-ramp ending abruptly where the span once began.

Mayor Brandon M Scott and Baltimore County Executive Johnny Olszewski Jr posted that emergency personnel were responding and rescue efforts were underway.

The Maryland Transportation Authority said: “All lanes closed both directions for incident on I-695 Key Bridge. Traffic is being detoured.”

The bridge, which opened in 1977, spans the Patapsco River, a vital artery that along with the Port of Baltimore is a hub for shipping on the East Coast. It is named after the writer of The Star-Spangled Banner.