Between Jan. 1 and June 30 this year, there were 841 reported cases of murder and non-negligent manslaughter, 7.7 percent more than in the period last year, when there were 781. In June, slayings rose 16.9 percent over June 1987, compared with a 4.1 percent increase in May.

''The murder figures scare me,'' Mr. Repetto said. ''And we still don't know what happened out there in July and August. But from reading the newspapers, an awful lot of shooting went on.''

Of the 841 killings in the first six months of the year, when murder and manslaughter went up 7.7 percent over the same period in 1987, there were 655 arrests, a 19.3 percent increase over the same period last year. Big Increase in Car Theft

For robberies, the number of arrests increased 4.1 percent, Chief Hoehl said, from 11,487 in the first six months of 1987 to 11,954 this year. For larceny theft, the number of people arrested increased 8 percent for the period, from 17,412 last year to 18,784 for the first six months of this year. The Chief noted that the increase in larceny arrests equaled the increase in the number of cases.

However, arrests for aggravated assault stayed relatively flat, dropping 0.8 percent, from 12,484 arrests between January and June last year to 12,384 so far this year.

The largest six-month increase in arrests was for motor-vehicle theft. Forty percent more suspects were seized this year than in the first six months of 1987, from 5,323 to 7,446.

Over all, police arrests increased in the first six months of the year over last year's period, Chief Hoehl said. The total reported crimes went up 8.4 percent over the first six months of the year, and arrests for all crimes went up 7.2 percent. Grim Picture by Koch