A man armed with an automatic pistol robbed Store 24 in Harvard Square of nearly $300 early Friday morning.

The robber entered the building at 1:45 a.m., ordered the five customers to move to the rear of the store and demanded that cashier Greg E. Higgens hand over all the money in the register. He then ordered Higgens to lie on the floor while he made his escape.

After the robber had fled on foot, the cashier called Cambridge police, who arrived almost immediately.

Cambridge police are now seeking a 5 ft. 8 in. white male with a brown mustache in connection with the incident, Sgt. Fred L. Centrella, head of the police investigation, said yesterday.

Store 24 will tighten its current security measures and consider additional ones in response to the robbery, the third since its opening two years ago, Jim L. Shiver-decker, store manager, said yesterday.

Red Flashes

At the time of the crime, Store 24's videotape equipment was not connected and its outside security light, which flashes red when activated from inside, was burned out.

In addition, none of the security guards employed by the Store 24 Company were present at the Harvard branch store that night.

In the future, the store may employ women to patrol the aisles with concealed cans of mace, the store's day-time cashier said yesterday.