The Montgomery County Council, in a move to stem a recent surge in condominium conversions, yesterday prohibited the county's apartment owners from converting any more units to condominiums for at least four months.

The emergency legislation, requested one week ago by County Executive Charles W. Gilchrist, was adopted unanimously. Gilchrist signed the measure immediately, putting it into effect, but reserved a decision on several other bills the council adopted to extend tenants' rights.

A similar conversion moratorium was rejected by the council last May at about the same time the D. C. City Council clamped a 90-day moratorium on condominium conversions. The D. C. law expires in one month but may be renewed.

The Montgomery County measure applies to all buildings that had not filed detailed conversion plans and proposed sales prices with the county by July 13. It also would prohibit building owners from entering into sales contracts or informing tenants of their intent to convert during the 120-day period.

In addition, the council passed legislation requiring owners to give their tenants or tenants' organizations 120 days to buy their apartments before the buildings could be sold to outside developers.

Gilchrist, in reversing his earlier opposition to a moratorium in Montgomery County, said last week that he had become alarmed on learning that 4,100 more rental units were scheduled for conversion in the county in addition to more than 2,500 apartments already turned over this year.

Supporters of the moratoriums argue that conversion movement, which is more active in the Washington area than almost anywhere else in the country, is drying up the supply of rental housing and imposing particular hardships on the poor and elderly.

Elderly and needy tenants are hardest hit because they are most likely to be uprooted when they cannot raise the down payments and combined monthly mortgage payments and management fees required for condominium owners.

A privately financed national market survey published earlier this year, before the moratoriums were proposed, estimated that the District and its suburbs would account for 13,000 conversions this year -- 10 percent of the national total and more than any other areas outside of New York and Chicago.

Two weeks ago, the county council defeated by one vote a measure giving tenants and the county the first right to buy their buildings after conversion. This was viewed as a more moderate provision than the legislation passed yesterday.

Gilchrist asked the council to await the results of a task force he was appointing before passing any legislation in addition to the moratorium.

Instead, the council passed the law, which will be in effect until Feb. 1, giving tenants and the county, acting on their behalf, exclusive rights for 120 days to purchase their apartment buildings.

It also adopted a measure requiring developers to allow professional engineers, hired by tenants, to inspect apartment buildings slated for conversion.

In addition, the council increased the county's rent supplements for the needy and raised eligibility levels to include families with incomes of up to $12,500 a year. That is $2,000 higher than current levels.

At public hearings during the last two days, tenants supported the moratorium and asked that it apply to buildings already undergoing conversion.

Representatives of the real estate industry described the moratorium as unnecessary but "the lesser of the possible evils."

"One hundred twenty days isn't going to kill us," said Terry Ryan, chairman of the condominium suncommittee of the Montgomery County Board of Realtors.

But other members of the industry have said the measure violates state law, and they have indicated they might challenge it in court.

"When a moratorium was proposed before, there was a great outcry against it," said councilman Scott Fosler. "Now everybody is saying it is the greatest thing since sliced bread. There was so much resistance to doing anything before, so here we are in what is called a crisis situation."

Council member Rose Crenca said industry testimony "depressed me more than anything I've heard since I've been on the council. Its heads you win, tails I lose."

She said the people involved in conversions say they don't want any interference as more and more tenants lose their homes.

The condominium movement was in its fancy in this area five years ago, when the District approved conversion of only 613 units, but it has sprouted more quickly in each succeeding year. Apartment owners most often blame rent control laws for making rental buildings financially unattractive.

Condominium conversion has spread into the Virginia suburbs as well, prompting battles in Arlington County and Alexandria, but those jurisdictions have no moratorium legislation currently pending.