The Montgomery County Council passed a rent control law yesterday that would prohibit annual rent increases of more than 10 percent in just over half of the county's apartments after April 1.

Passed as an emergency bill on a 6-to-1 vote, the law establishes the first mandatory ceilings on rent increases since rent control was lifted by a previous County Council in December 1977.

The measure applies only to apartments occupied by their present tenants before July 1, 1976. Industry spokesmen said this measure affects no more than 26,000 of the 46,000 units in Montgomery County.

The measure will automatically expire on Jan. 31, 1981.

"We must be the only jurisdiction in this entire nation going back to rent control," said council member Esther Gelman, the only dissenting vote. "It's counter to the national mood."

The bill was introduced only three weeks ago by new County Executive Charles W. Gilchrist, who said he was acting on a campaign promise to prevent "rent gouging of a substantial number of tenants."

"Mr. Gilchrist has kept every promise to the letter," tenant leader Marilyn Wisoff said yesterday, after the vote.

The law asks landlords to voluntarily hold rent incrases to 7.6 percent and requires that any increases above 7.6 percnent be justified. The landlord must prove to the Office of LandlordTenant Affairs that rising costs require the increase.

The 7.6 percent voluntary guideline may change as the cost of living goes up or down in the Washington area. Reviews are required every six months.

But increases above 10 percent are strictly forbidden unless they are granted by the Office of LandlordTenant Affairs under extraordinary circumstances, such as to ecompensate for soaring fuel costs.

Unlike a voluntary measure last year, the new law permits the land-lord-teant office to force a rollback of "unjustified" rent increases.

As an emergency bill, Gilchrist's proposal needed approval by five of the seven council members. He apparently always had the support of Elizabeth Scull, a sponsor of more strict rent control legislation; Scott Fosler, Neal Potter and Rose Crenca.

Through intensive lobbying during the two weeks of marathon rent control hearings and work sessions, Gilchrist's staff won the suppirt if Ruth Spector, Gilchrist's former legislative aide in Annapolis. She originally opposed the measure.

Spector said yesterday she changed her mind after she won unanimous agreement for an amendment to control only the 26,000 apartments in which the same tenant has lived since before July 1, 1976.

In the end, council member Michael Gudis, who had been wavering, voted for the bill.

Yesterday Gelman successfully sponsored an amendment to exempt apartments renting for$600 or more a month. Landlords said that affacts about 100 units otherwise controlled by the law.

Caroline Lewis, lobbyist for the Apartment and Office Building Association of Metropolitian Washington, called the law a "giant step backward" and said it was inconsistent with the county's aim for more moderate cost housing because it could impede new apartment construction.

But landlords' previous threats of a rash of condominium conversions because of the proposal were considerably muted yesterday.

Some property owners were smiling after the vote because they said it virtually gives them license to raise rents as much as 10 percnet and at the same time exempts nearly half the county's apartments.

About 6,000 new apartments are in some phase of construction in the county.

The former council abandoned a 5 percent rent ceiling at the end of 1977 and established a voluntary limit of 6.1 percent during 1978.

Gilchrist argued that under the voluntary law, at least 12 percent of the tenants, many of them elderly people on fixed incomes, "suffered from excessive and unwarranted," increases above 10 percent, and in a few cases, as much as 50 percent.

At least 13 percent of the rent increases scheduled for April 1 are higher than the 1 percent ceiling imposed by the new law.

One tenant who asked that his name not be used said his rent was scheduled to go up 15.2 percent, to, $364 a month, on April 1. "That is a shocking situation for people that live on a fixed income, whose pension last year went up by only 4.9 percent," he said.

His increase, county officials said, will be subject to the new law.