Heavily armed Montgomery County police officers evacuated about 120 customers and employes from a Rockville Pike clothing store last night and, using at least two K-9 dogs, searched the premises for an armed robbery suspect without success.

About an hour after police abandoned their search, an internal alarm was triggered in the Syms Inc. store at 11840 Rockville Pike, and police returned and arrested a man who they said emerged from a back door, according to police spokesman Phillip Caswell.

Caswell said police believed the man had been hiding in the store, but that police had "no idea" just where.

The incident began shortly before 5 p.m., when a man with a pistol took cash, jewelry and credit cards from a man at Randolph Road and Glenallen Avenue in Wheaton and drove off, police said.

Later, employes at a Rockville Pike stereo store became suspicious of a man who tried to make a credit card purchase. Police were called and given his description.

Shortly afterward, police said, a man who appeared to fit the description was seen near the Syms store and chased into it.

Police serched the store, but gave up at about 8:40 p.m. and said they believed the suspect no longer was there.

Police identified the man arrested after the store's alarm sounded as David Milton Blackstock, 26, of no fixed address.

Caswell said that Blackstock was charged with armed robbery and the use of a handgun in the commission of a felony.