An early morning two-alarm fire coupled with an explosion of undetermined origin yesterday at Oxon Hill's Dennis Grove Apartments left 26 families temporarily homeless and nine persons injured.

For several hours as a result of the blaze, residents of the entire 840-unit complex of garden apartments were without heat and gas and, for a short time, without electricity as firefighters worked to bring the fire under control.

Nine engine companies, three truck companies, five ambulances and a medical unit responded to the two fire alarms at 2:24 and 2:31 a.m. A section of the damaged building at 2208 Alice Ave., collapsed while firemen were fighting the blaze.

It was not immediately clear whether the explosion preceded the fire. The blast, according to fire department operations officer Bert Shaffner, literally blew three persons from their apartments into the street.

Thelbert H. Haskins, 32, was thrust by the explosion through his window and found lying on the street, officials said. He was transported by private automobile to Greater Southeast Community Hospital with a broken leg. Six other residents and two firemen were taken to the hospital with minor injuries and then released.

Fire officials estimated damage to the building at $300.000, not including contents of the various apartments.

Firefighters were hampered in battling the blaze by flames that melted the outside gas meter, allowing leaking gas to feed the fire. Gas valves were frozen, requiring ground to be dug up in order to reach the main cutoff switch for the entire complex.

Ziggy Shellick, of Zalco Realty, the apartment project's management firm, said the 26 families made homeless by the fire were being moved into unoccupied units in the complex. Fire department investigators, meanwhile, were sifting through the wreckage to determine the cause of the blaze.