A five alarm fire that investigators said was deliberately set destroyed a historic church in Porter Square early yesterday morning.

More than 25 Cambridge fire vehicles responded to the 4 a.m. alarm. It took fire fighters nearly six hours to control the blaze, fireman Donald J. Warren said yesterday.

Firemen, fearing that the church's gutted steeple would collapse, cordoned off the area before a crane could knock the steeple down safely.

"There is no question that the fire was deliberately set," Lt. John R. Ambrogne, a detective from the state fire marshal's office who is investigating the incident, said yesterday. "There was some type of ignition device underneath the front porch," he said.

Police do not know exactly what was used to start the blaze and there are still no suspects in the case, Ambrogne said.

"There are usually a lot of pranks on Halloween, but most of them are false alarms," Warren said.

The Cambridge Historical Society recently declared the Cornerstone Baptist Church, built in 1853, a historical landmark. The church was up for sale.