The Department of Housing and Urban Development agreed yesterday to accept bids for $500 million worth of low-income rental housing mortgages but to not actually sell the loans until July to give Congress and activists time to block the sales.

HUD has scheduled auctions of the loans for today and Friday, with prospective buyers asked to deposit "earnest money" of 10 percent of the unpaid balance of the loans on which they are bidding.

Some of the mortgages are federally insured loans HUD took over after the property owners defaulted, and others are loans made by the department in order to sell properties it had taken back after previous defaults. In most cases, low-income residents of the buildings will not be protected against rent increases after the sales, the critics said.

Four of the projects involved in the auction are in the Washington area. They are the Rockview Apartments, Hartford Terrace and Waldron Apartments, all in the District, and Leesburg Manor Apartments in Leesburg.

Congressmen and housing organizations are becoming more vocal in their concern about the possible loss of thousands of units from the low-income housing stock. Within the next 15 years, more than half of the 600,000 privately owned, federally subsidized apartments in the nation could be freed from the regulatory restraints that ensured their continued use as low- and moderate-income housing, according to several estimates.

Market conditions, tax reform and budget cuts over the past six years have eliminated nearly all new construction of housing for the poor, housing advocates said.

The National Housing Law Project, a nonprofit organization, filed suit Friday in U.S. District Court for the Northern District of California, and announced plans to ask the court to issue a temporary restraining order yesterday halting the auctions.

HUD and the nonprofit group, however, agreed yesterday that when the department accepts offers it will tell bidders the sales will not be finalized until July 8, and that pending legislation and the outcome of the legal action could cancel the transactions, according to Catherine M. Bishop, an attorney with the Berkeley office of the Housing Law Project. In return, the organization will not request a restraining order now, she said.

The delay in closing the deals "will give the court time to rule on our request for a preliminary injunction against the sales," Bishop said.

The lawsuit says that the sales as the administration has planned them would cancel regulatory agreements that control rents and contain other tenant protections, according to Bishop. In selling the mortgages, HUD is "thwarting" laws that place limits on such sales, she said.

Federal Housing Commissioner Thomas T. Demery said tenants in the housing projects "are going to remain protected. . . . All we are doing is changing who the {building owner} pays the mortgage to." HUD "will not allow" low-income residents to face rents beyond their ability to pay. He said that "once the facts are heard" during court proceedings, "everyone will be assured that the people will be protected."

HUD's agreement to delay closing the sales is the second time in a month the administration has backed away from selling low-income housing project mortgages. In late April, HUD, at Demery's urging, called off plans to sell securities backed by $410 million worth of mortgages on rental housing projects. Demery said yesterday that he "urged deferment" of the April auction in hopes of obtaining promises of tenant protections from the buyers of the loans.

Seven members of the House housing subcommittee signed a letter urging Demery to cancel this week's sale as well.

A housing authorization bill approved in late May by the House Banking, Finance and Urban Affairs Committee contains provisions limiting the administration's ability to sell subsidized properties without restrictions to keep the rents low, but the legislation passed by the Senate in March has no such limitation.

The housing and community development act Congress hopes to pass later this year, however, is expected to have "provisions to retain the low-income affordability of federally assisted units while protecting the rights of owners," according to a letter sent to HUD this week by Alan Cranston (D-Calif.), the Senate housing subcommittee's chairman, and Alfonse M. D'Amato (R-N.Y.), the panel's ranking Republican.

The timing of this week's scheduled sale "is particularly unfortunate" because the Senate subcommittee plans hearings Friday "to explore the potential loss of thousands of low-income units through mortgage prepayments and expiring rental subsidies," according to the letter.

Sales of federally subsidized housing mortgages is part of the administration's $5 billion loan asset sales program, designed to help shrink the budget deficit. The program, however, is months behind schedule and is controversial. Cranston and D'Amato said the government "would take a real loss to show a short-term accounting gain" because the sale proposed by HUD would be made at "deeply discounted prices."

Loans being auctioned today will be discounted as much as 20 percent off the unpaid balances, and those sold Friday will have no discount limits.