As many as one-quarter of the tenants in Montgomery County probably received rent increases in January following the ending of rent controls the month before, the head of the county Office of Landlord-Tenant Affairs said yesterday.

Nearly half of the increases on which the office has information exceeded the suggested ceiling in effect under the county's new voluntary guidelines.

A tenants' representative called the increases "outrageous but expected," while a spokesman for the building owners defended the increases as necessary to cover increased costs.

Approximately 2,000 notices of rent increases have arrived at the landlord-tenant affairs so far, said acting director Elise Hall. She said she expects another 8,000 to 10,000 to arrive in the next week or two. The office oversees rents for about 47,000 rental units in the county.

Under the voluntary guidelines law that replaced rent control, landlords are required to give at least 60 days notice for increases, which means they had to notify tenants by Jan. 30 about increases that would become effective April 1.

The landlords also must notify the landlord-tenant affairs office about rent increases within a reasonable time.

The office has no legal authority to take action against landlords who exceed the guidelines, but the County Council has warned that large-scale vi olation of the guidelines might result in a reinstitution of rent control.

About 50 landlords have reported increases so far, said Hall, but the most complete data she has is for 27 landlords who reported by Jan. 20.

They accounted for 729 increases, she said, and 57 percent of those were at or below the guideline figure. A corresponding 43 percent exceeded it.

The "typical" rent increase for those landlords was 8.9 percent, Hall said, but there were a number of increases in the 15-to-25 percent range, and, in a-few instances, increases as high as 50 percent and 78 percent. Those extraordinarily high increases, Hall said, generally occured with units that had been under rent control for such a long that their rents were far below market level.

Ruth Lederer, president of the Montgomery County Tenants Association, said the landlords were "taking advantage of a situation." She said she "had the feeling that once rent controls were off, many people were not going to practice prudence." she said.

John O'Neill, executive vice president of the Apartment and Office Building Association of Metropolitan Washington, which represents about two-thirds of the county's building owners, said he found assertions that so many were violating the guidelines "questionable."

Under the new law, he contended, owners will conform to the guidelines if the average of all their increases in a particular project does not exceed 6.1 percent. They do not have to keep the increase for every unit within that figure, he said.

"I think what we're hearing is complaints from tenants whose rents are being equalized," he said.

The County Council recently issued a report stating that the council's interpretation of the law was that "if the overall increases stay within the guideline, the landlord will have conformed to the guidelines." The Office of Landlord-Tenant Affairs, the agency charged with administering the law, has not yet decided whether it will interpret the law that way. Hall, who took over as acting head on Monday, said she hopes to make a decision by the end of this week.

John Menke, the council member who spearheaded the move to voluntary guidelines, said he was pleased that "the average increase is at least somewhat close to the guidelines."

Council President Elizabeth Scull, who voted against getting rid of rent control, said it was too early to say whether the council might bring back rent control. "We haven't yet begun to exhaust all the procedures fordealing with the situation," she said.