BOSTON — A 6-alarm arson fire tore through a large apartment building, including the apartment of a couple on the third floor who dropped their young grandson into the arms of a waiting firefighter, officials said.

A man who came to a hospital with burns and claimed he started the fire was arrested, police said.

Firefighters rescued at least 15 people, including the 6-year-old grandson of Herbert and Judith Lamb, who live on the third floor of the apartment building.

The Lambs broke a window and dropped their 6-year-old grandson into the arms of waiting Boston firefighters.

Boston Fire Lt. Glenn McGillivray, who caught the child, said, “I knew she wouldn’t be able to hold him until we got help, so they dropped him, and I caught him in my arms from the third floor.

“Thank God I caught him. I’ve never had to do that before, and I hope I never have to do it again.”

He said firefighters used a ladder to rescue Judith Lamb.

The blaze damaged or destroyed more than two dozen apartments in the city’s Roxbury neighborhood.