Education Secretary William J. Bennett yesterday officially unveiled the administration's controversial new plan to provide federal money in the form of vouchers to allow about 5 million low-income children to attend private or public schools outside their assigned districts.

In a news conference, Bennett defended his plan against charges -- from teachers' unions and education lobbyists -- that it would undermine the public school system while getting the government into the business of paying a poor child's tuition to attend religious schools.

Bennett compared his voucher plan to the federal Pell grant program that provides money to low-income college students to attend the public or private universities of their choice.

"It will increase educational opportunities by creating real opportunities -- by creating choices -- for the parents of the educationally disadvantaged," Bennett said. "At present, our more affluent families do exercise choice, by buying a home in a certain neighborhood of their choice, or by sending their children to a private school."

The plan would allow parents who qualify for Chapter I (formerly Title I) education aid to opt to receive the money directly via a federal voucher. Currently, the money goes directly to school districts, which use it to set up enrichment or remedial programs.

Chapter I, 20 years old, is the largest area of federal involvement in education, consuming almost a quarter of the Education Department's budget.

Opposition to Bennett's proposal began crystallizing before its announcement, and yesterday Bennett responded with a list of the plan's supporters. They include Sen. Orrin G. Hatch (R-Utah), who will sponsor it in the Senate, the Lutheran Church-Missouri Synod, the National Catholic Educational Association and Agudath Israel of America, an Orthodox Jewish group.

The administration proposed a similar voucher plan in 1983, which died in Congress after Republicans and Democrats opposed it, and Bennett made no predictions about this plan's chances. "I don't think one should ever underestimate the power of a good idea," he said.

Education Deputy Undersecretary Gary L. Bauer, the department's point man on this and other conservative issues, said he would devote a considerable amount of time to lobbying for the plan on the Hill. And Hatch, chairman of the Labor and Human Resources Committee, said in a statement, "I guarantee that this innovative and sensible approach will have high priority for serious consideration by the United States Senate."

Bennett spelled out differences between this plan and past voucher proposals. The 1983 plan gave local authorities the option of participating in a voucher plan, for example, while this proposal gives the choice to every parent who is eligible for Chapter I aid.