President Carter announced plans yesterday to step up the government's attack on youth unemployment, by strengthening existing job programs and by underwriting coordinated school-and-work programs in poverty areas.

The election-year proposal, which will be the major domestic initiative in the fiscal 1981 budget Carter will send Congress Jan. 28, eventually would add $2 billion a year to the $4 billion spent on federal youth programs.

However, in line with Carter's tight budget policy, the White House is proposing to phase in the program, alloting only $150 million in planning funds for fiscal 1981 and $1.33 billion in fiscal 1982.

The proposal was unveiled with an unusual amount of fanfare, including special briefings for interested constituent groups and a full-dress ceremony in the East Room with a brief address by the president.

In a speech billed as "major" by key aides, Carter termed the effort "a historic investment in our nation's most underused resource." He called the proposal "the most comprehensive youth and training program ever envisioned."

The new program would be divided into two major elements:

For youths aged 18 to 21, the plan would consolidate three programs that provide part-time jobs and training and impose tough standards to weed out participants who don't make the grade.

For those aged 14 to 18, the government would provide aid to local school districts to develop back-to-school programs that coordinate remedial education, vocational training and after-school job experience.

The money would be distributed according to a formula to the 3,000 poorest of the nation's 15,000 school districts. Each individual school involved would have to submit plans outlining its proposal in advance.

The changes would be designed to correct a few of the most significant flaws manpower experts have found in existing youth programs -- the lack of coordination with local schools and the absence of standards for participants.

The jobs would be provided either by local employers, using existing government, subsidies, or by county or municipal, governments, as is being done in current programs. There would be no additional hiring incentives for companies.

Officials estimated yesterday that the program, when fully in force, would serve 1 million more youths on top of the 2 million now participating in existing work and training programs.

Ray Marshall, the secretary of labor, said the White House has estimated there are 3.5 million youths who will need education or training each year. Many are black or Hispanic and nearly all are from poverty areas. e

The jobless rate for black youths is about 33 percent, compared to 14 percent for white youths, but the actual number of youths out of work is relatively small.

Labor Department figures show there are about 1.5 million youths unemployed (in a total U.S. workforce of 103 million persons). Of those youths, 1,196 million are white. Unemployment among black youths totals 332,000.

Coincidentally, Carter's initiative comes at a time when experts say the population "bulge" is declining and the number of youths in the population, both employed and unemployed, is shrinking each year.

However, administration planners argue the step-up is needed anyway because, while overall youth unemployment will be declining, the number of black and Hispanic youths out of work will rise over the next few years.

The White House also denied that the move was designed as an antirecession step. Stuart Elizenstat, Carter's chief domestic adviser, said the increase "needs to be done apart from what happens to the unemployment rate nationally." "

The new Carter proposal immediately was endorsed by a spate of key constituent groups, from black organizations to mayors' groups and labor unions. The administration had consuited closely with each during the planning stages.

It was not immediately clear how the proposal would be received by Congress. However, key officials were optimistic. Youth programs consistently have received strong support from both parties.

The plan was developed by an interagency task force headed by Vice President Mondale. The panel made a nine-month study of the youth unemployment problem, which Carter earlier had labeled as a major social target.