President Reagan and congressional leaders, concluding a month of intensive and often divisive budget negotiations, yesterday announced agreement on the outlines of a plan they said would reduce the federal deficit by about $76 billion over two fiscal years.

"We are sending the right message at the right time," Reagan said as he announced the accord at the White House with congressional leaders arrayed behind him.

The president called the agreement "a blueprint that sends a strong signal both at home and abroad that together we can and will get our deficit under control and keep it that way."

However, the budget plan agreed to yesterday sketches out only in broad detail the combination of spending reductions and revenue increases needed to meet the deficit-reduction goals.

The fine print of which taxes will be raised and which programs will be cut is to be fleshed out later, and could prove to be almost as controversial as achieving the accord.

In a tacit concession that the budget accord faces considerable opposition in Congress, where House and Senate Republicans have denounced it as too modest and too dependent on tax increases, Reagan promised that he and congressional leaders will "roll up our sleeves and go to work so we can complete this important job."

The agreement, which Reagan said would yield a "credible and reliable" reduction in the nation's high budget deficits, was reached one month and a day after the Oct. 19 stock market plunge sent tremors through world financial markets.

The aftershocks of the 508-point drop in the Dow Jones Industrial Average of 30 stocks that day brought the administration and Congress to the negotiating table amid an international clamor that Washington come to grips with the deficit.

Whether the outline announced yesterday will restore confidence in the world's stock markets is uncertain.

Reagan yesterday showed disdain for those on Wall Street who have complained that the budget plan does little more to attack the deficit than would have been automatically achieved by the Gramm-Rudman-Hollings budget law, even without an accord.

Responding to a question about Wall Street's concerns, Reagan paraphrased a letter he said he had received recently in which an unidentified correspondent observed, in the president's words, that "even a farmhand cleaning out the stalls in a barn knows that what he's cleaning out didn't come from outside. It was produced in the barn . . . . They've {Wall Street leaders} got some things to straighten out themselves also."

The agreement calls for $30.2 billion of deficit reduction in this fiscal year and $45.9 billion in fiscal 1989, which begins next October.

The plan stipulates $9 billion of unspecified taxes in the first year, and $14 billion in the second year. The negotiators have agreed only on what they will not be: changes in income tax rates, sales taxes, or alterations in income tax indexing. What they will be won't be known until tax-writing committees craft a bill.

On the spending side, some reductions are spelled out.

The $4 billion in entitlement reductions in the first year, for example will be divided as follows:$2 billion from the Medicare program.

$900 million from farm subsidies.

$250 million from guaranteed student loans.

$850 million from changes in pay grade structures for federal employes.

But like the taxes, the $2.6 billion reduction in discretionary domestic spending cuts, as well as the $5 billion cut in defense spending, will have to be translated into specifics by congressional committees.

Some other savings are more concrete:$5 billion in one-time reductions to be achieved by refinancing loans held by rural electrical cooperatives.

$1.6 billion from more vigorous income tax collections.

$1.2 billion ostensibly will be saved from the lower interest costs on the national debt.

$400 million in user fees must still be spelled out.

Reagan yesterday played down his significant retreat from a position he had stubbornly clung to through most of the year: that a tax increase would be enacted "over my dead body."

The president reminded reporters that his budget also asked for some tax increases, a point Democrats had made repeatedly as they skirmished with the administration over budget plans for nearly 10 months before "Black Monday" on Wall Street.

The accord was reached after congressional leaders and administration representatives agreed on language designed to bind congressional committees to the two-year schedule of spending reductions, a concession to the administration, which has repeatedly charged Congress with reneging on agreements to cut spending.

Top congressional leaders also hailed the agreement -- reached after 20 days of talks stretched over four weeks -- as a watershed event that marks a departure from the fiscal warfare that has often marked relations between Capitol Hill and the White House during the Reagan presidency.

House Majority Leader Thomas S. Foley (D-Wash.) called it a "milestone in our efforts to bring about a reduction of the deficit."

"This is truly a bipartisan agreement," House Speaker Jim Wright (D-Tex.) said. "Nobody gets everything he wants . . . . It is a demonstration that in time of stress, the administrative and executive, as well as the legislative branches of government, can work together, even when they are in the hands of different political parties."

The agreement would yield only slightly more in the first year than would be achieved by automatic cuts under the Gramm-Rudman-Hollings balanced-budget law, which mandates $23 billion in budget reductions in the absence of any alternative agreement between Congress and the White House.

Reagan late yesteday signed an executive order implementing Gramm-Rudman Hollings, and the first, minor cuts under the budget law were to take effect at midnight. The White House refused to delay the onset of the cuts and it was clear there weren't sufficient votes in Congress to postpone them.

However, if the deficit accord is enacted by Congress in separate legislation that will be taken up after Thanksgiving, the automatic cuts would be superseded by the agreement.

Passing that legislation remains a significant hurdle, but one that congressional leaders predicted could be surmounted.

"Hopefully we can sell it," House Minority Leader Robert H. Michel (R-Ill.) said. "Hopefully we'll convene in the first part of December with a little better attitude."

"We are pledged to give our support" to the "full implementation" of the plan, said Senate Majority Leader Robert C. Byrd (D-W.Va.), who greeted the accord as a "very good birthday present" on the day he turned 70.

Supporters of the agreement predicted that most lawmakers who have opposed the plan at first blush would change their views once they take a sober look at the alternative of across-the-board cuts that would slash $11.5 billion from defense and a similar amount from domestic discretionary programs. Politically sensitive programs such as Social Security are exempt from the Gramm-Rudman-Hollings cuts.

Any chance of a more ambitious program of deficit reduction slipped away early this week when negotiators abandoned their frequent discussions of freezing or delaying cost-of-living increases for Social Security recipients, federal workers and government retirees.

The budget accord will not become law until Congress approves, and Reagan signs, two pieces of legislation that will be taken up in early December. The first would incorporate the tax increases and spending reductions in entitlement programs, such as Medicare, that dispense benefits based on existing law. The second would be a catchall appropriations bill to fund the government and would include the proposed reductions in discretionary spending for defense and domestic programs.

The taxes are likely to be drawn from a pool of about $18 billion in higher levies included in separate House and Senate tax bills. The House has already passed its version, and the Senate legislation has been reported out by the Finance Committee. The two bills have about $8.4 billion in common taxes, most of which are aimed at wealthy individuals and corporations.

Meanwhile, Treasury Secretary James A. Baker III indicated that a meeting of the seven major industrial nations to discuss global economic policy would have to await progress in moving the budget accord through Congress.

Some financial analysts had expected Baker to convene such a meeting shortly after an agreement in principle was reached on the budget. Baker hopes to persuade West Germany and other U.S. trading partners to stimulate their economies in order to reduce the risk of a worldwide recession.

But, Baker said, "We have an implementation to perform here" on the budget. The Treasury secretary said U.S. officials have been in touch with allied government leaders, but he added: "We need to devote our efforts over the next few weeks to implementing this {budget} agreement so we can deliver if there is" an international meeting.