Mayor Marion Barry has written to the merchants hit by vandals during last Saturday's anti-Klan outburst to tell them he's sorry they were the victims of "a senseless and sorrowful expression of disruption . . . . "

But the mayor, in a letter dated Monday and signed in his absence from the city by City Administrator Elijah B. Rogers, did not offer any compensation for the damage suffered by the 23 businesses in the areas generally east and north of McPherson Square. They included a pawnbroker, a bank, a cafeteria, a bike shop, clothiers and a saloon.

" . . . I strongly hope that you will not permit the memory of these intolerable acts to supply cause to lose faith in our community," the mayor's letter said. "Please accept my prayers and hopes that you will be able to swiftly resume your operations . . . . "

Police Chief Maurice T. Turner Jr. has made personal visits to the merchants in the company of U.S. Attorney Stanley S. Harris. Barry's letter and Turner's visits were not announced at city hall, so you can't write them off as a public relations event. In Barry's excessively image-conscious administration, that's a plus.

"We were real pleased to get the letter," said a manager of one of the businesses. "We thought it was a nice gesture."