The Montgomery County Housing Opportunity Commission yesterday opened the way for 500 moderate-income, first-time home buyers to obtain mortgages with rates of 10.78 percent--about three points below the going rate on home mortgages in the area.

The money to finance the mortgages comes from the sale last week of $32 million in tax exempt mortgage revenue bonds, a commission spokeswoman said. The sale marked the sixth time in recent years that the commission has sold the bonds to provide such loans below the going market rate, according to the spokeswoman. In the past, the sales have helped more than 3,000 local residents purchase homes, said spokeswoman Joyce Siegel.

The houses available for the 30-year, fixed-rate mortgages are located in 26 new home subdivisions in the county and are priced below $70,000, Siegel said. The homes are mostly town houses, although some condominiums and single family detached houses are among those offered.

To be eligible for the program, prospective buyers must have incomes within this range: $27,900 maximum for a household of one; $32,900 maximum for a two-member household; $33,900 for a three-member household; $34,900 for a four-member household and $35,900 for a household of five or more.

The state of Maryland and Prince George's County have offered similar programs in the past.

This offering could be the last made by the commission because, under current federal legislation, the program is to come to a halt at the end of this year, Seigel said.

Opponents of the program, including the Reagan administration, see it as a tax revenue drain, though estimates vary widely on how many tax dollars are lost through the program. Opponents estimate a $300 million tax drain in 1982, while proponents of the program say the loss was less than one-third that size.

Commission executive director Bernard L. Tetreault has said that the program is "the only way a county like Montgomery has any hope of maintaining any sort of economic integration."

Tetreault heads a national organization of local housing financing agencies attempting to extend the life of the program.

Potential buyers must go directly to the subdivision sales offices and will be sent from there to one of the participating local lenders to apply for a loan. The commission has set up a telephone line to channel potential buyers to the right locations: 565-HOME.

A listing of the subdivisions offering homes in the program will appear in the Maryland Weekly on Thursday.