Major crime in Montgomery County decreased in 1981 for the first time in five years, despite an almost 20 percent increase in crimes against persons, particularly armed robberies and aggravated assaults, Police Chief Bernard Crooke said yesterday.

Burglaries led the decline, as the police department concentrated on the large number of residential and commercial burglaries that have been plaguing the county. Crooke said the number of such burglaries dropped 18.2 percent for homes and 7 percent for businesses in 1981.

Drunk driving also was a department priority, with arrests increasing by 95 percent. A total of 1,464 people were arrested for drunk driving in 1981. Police arrested 751 people on drunk-driving charges a year earlier.

Changes in police operations, such as placing investigators in district offices, rather than headquarters, helped increase the number of arrests for serious crimes by 14 percent in 1981, Crooke added. Arrests for serious crimes increased to 5,364 from 4,704 in 1980, police statistics show.

The county's Special Assignment Teams, small groups of plain-clothes officers in each district who look out for suspicious activity, were responsible for 992 arrests from last March through December, including several for crimes in progress, Crooke said.

Crooke said that burglaries and robberies will continue to be a priority in 1982. "We are concerned about robbery increases," Crooke said. Robberies increased by 9.3 percent, to 872 incidents from 798 a year earlier.

County Executive Charles Gilchrist said efforts will be made this year to bring the police force closer to its authorized strength of 780 officers. Currently, the force numbers about 740, but in recent years has numbered 720.

Crooke added that the county's precious metal law, implemented in January, 1981, helped investigators recover $265,305 in stolen goods. The law requires dealers to make police reports on any gold, silver and other precious goods they purchase