Serious crime continued to decline in Montgomery County for the first half of 1982, with the greatest decreases in armed robberies and residential burglaries, according to statistics released yesterday.

The figures show that from January through June there were 207 armed robberies--25.3 percent fewer than the 277 during the first half of 1981. The 1,960 residential burglaries were 20 percent fewer than the 2,454 in the first six months of last year.

Total crime declined by 8.7 percent compared with last year. Fourteen persons were slain during the first half of 1982, compared with 8 last year--an increase of 75 percent. Aggravated assaults increased by 28.5 percent--415 this year, compared with 323 the year before. Police Chief Bernard Crooke attributed the increase partly to the late May incident at the IBM building in Bethesda, in which a masked gunman killed three persons and wounded several others during a daylong rampage.

Overall, Crooke said, the latest figures showed "a complete turnaround from what we've been experiencing . . . What has been accomplished in Montgomery County is good. It doesn't mean the battle is won." He attributed the drop in burglaries in part to passage of a law restricting sale of precious metals and special police effort.

Crime in Montgomery has become the subject of some political debate in this election year, and some candidates for county executive, particularly Republican Luiz R. Simmons, have accused the incumbent, Democrat Charles W. Gilchrist, of ignoring the crime increases during the first two years of his term.

Crooke said the criticism "borders on being irresponsible. We publicized the figures when we were on the increase, and we're publicizing them now that they're down."