Serious crime in Arlington rose by 2.2 percent in the first six months of this year compared with the same period in 1984, reversing a trend of declining crime rates in recent years, police said yesterday.

The number of reported serious crimes -- murders, rapes, robberies, aggravated assaults, burglaries, larcenies and vehicle thefts -- rose from 3,589 to 3,668.

The largest increases were in robberies and aggravated assaults, while the number of murders, burglaries and vehicle thefts dropped.

The number of rapes and larcenies rose slightly.

Police spokesman Tom Bell, while noting that officials rarely can explain short-term fluctuations in the crime rate, said the dramatic decreases of recent years may be starting to level off.

In 1982, serious crime in Arlington declined 17 percent; it dropped 4 percent in 1983 and 0.2 percent last year.

"It's very possible that it bottomed out," Bell said. He said many aggravated assaults arise from domestic quarrels and there is no pattern to explain their increase from 169 in the first six months of 1984 to 195 this year.

"There is no common denominator with the robberies either," according to Bell, who noted that the increase from 129 to 149 cannot be attributed to a cluster of incidents in one place or a series committed by the same person.

Police departments in Montgomery, Prince George's and Fairfax counties, Alexandria and the District of Columbia have not released statistics on reported crime for the first half of this year.

During the first quarter of the year, police in Arlington, Prince George's, Montgomery and Fairfax counties reported increases in serious crime.

In Alexandria, serious crime decreased by about 0.5 percent, and in the District, it fell by 11 percent. The District figures included an 8 percent increase in arson -- a category not reported in the suburban jurisdictions.

There have been two murders this year in Arlington, compared with four in the same period last year, and 24 rapes, compared with 23 a year ago.

Burglaries declined 1.7 percent, from 724 to 712, and vehicle thefts dropped 0.8 percent, from 255 to 253. Larcenies rose 2.1 percent, from 2,285 in the first half of 1984 to 2,333 so far this year.