Congressional Democrats moved yesterday to thwart President Reagan's plan to get the federal government out of the housing field, as a top industry official warned that a housing collapse is imminent.

A House appropriations subcommittee has rebuffed administration plans to cut $9.4 billion from this year's budget by recapturing large numbers of federally subsidized housing approved last year.

The administration has counted on not building or rehabilitating a number of low-income and elderly housing projects that already had been funded, but the subcommittee on HUD and related agencies yesterday merely agreed to defer some of the spending from the fiscal 1982 to the fiscal 1983 budget.

About 50,000 planned units of housing would be maintained by the panel's action, a staff aide said.

Housing-start figures released yesterday showed a 6 percent increase from a month earlier, but neither the administration nor the industry considered the slight rise a sign of an imminent recovery for housing.

The House Banking subcommittee on housing started hearings yesterday on a comprehensive housing plan by the Democrats, and members of the traditionally Republican housing industry testified that they must have federal help to avoid a catastrophe.

"My message today is simple and clear: The housing collapse that we feared but prayed would not occur is upon us," said Frederick Napolitano, president of the National Association of Home Builders.

The homebuilders support a mortgage interest subsidy plan offered by Republican Sen. Richard Lugar of Indiana, chairman of the Senate housing subcommittee.

Senate Democrats, meanwhile, are preparing their own comprehensive housing legislation along the lines of the House Democrats' bill for new construction, home ownership and low-income housing aid. This legislation will be introduced later this week or next week, a staff aide said.

The administration itself is reassessing its housing policy now in light of the continuing depression in the industry, but administration sources said it still would not consider anything with a high price tag including the Lugar bill.

"Housing and autos have to be counted on to lead the recovery," one White House official said. If Reagan were to shift at all from his reliance on the economic recovery program already in place, "it would be to provide some short-term stimulus to these areas," the official told reporters on the basis that he not be indentified.

Housing starts in February rose 6 percent from January's level to 953,000 units on an annual basis, pulled up by an increase in condominium starts while single-family home starts dropped. The February figure was 26 percent below the rate a year earlier. Building permits, an indication of future construction, stayed flat from the month before at 838,000 units.