WASHINGTON--President Reagan and congressional leaders yesterday announced agreement on a two-year, $76 billion deficit-reduction plan designed to meet the goals of the Gramm-Rudman budget law and reassure jittery financial markets.

The agreement was reached a month after the Oct. 19 Wall Street plunge which shook Washington and prompted 20 days of strenuous negotiations on Capitol Hill between Republican and Democratic congressional leaders and representatives of the administration. Until the last minute, Republicans were balking, unwilling to go along with $23 billion in new taxes over two years.

Despite the agreement calling for spending cuts, tax increases and sales of government assets, Reagan ordered Gramm-Rudman's $23 billion in automatic across-the-board spending cuts to begin.

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

"This agreement is probably not the best deal that could be made, but it is a good, solid beginning," Reagan said. He said it was "a plan that meets our short-term concerns and while laying the foundation for longterm solutions."

"It is a balanced package. Everybody gives some, nobody gets everything he wants. Not the president, not the Congress, not Democrats nor Republicans," said House Speaker Jim Wright (D-Texas). Wright and other Capitol Hill leaders joined Reagan at the White House in making the announcement.

But lawmakers said it would be an uphill fight to win approval next month of the spending cuts and tax changes envisioned in the agreement.

Most of the Gramm-Rudman cuts would not be felt immediately, as federal agencies already were juggling accounts to avoid major disruptions such as furloughs or layoffs. Part of the agreement is that Reagan would minimize the impact of the Gramm-Rudman cuts while the necessary enabling legislation is being produced.

Some of the cuts--including an 8.5 percent drop in dairy price supports and a 2.3 percent cut in Medicare payments to doctors and hospitals--would be implemented as soon as Reagan signed the order.

However, if the deficit-reduction agreement is enacted, the Gramm-Rudman cuts would be canceled and programs would be restored.

The agreement would reduce the deficit in fiscal 1988, which began Oct. 1, by $30.2 billion, to about $149.7 billion, based on congressional estimates. Both the administration and congressional estimates show the deficit shooting up from last year's $148 billion if no action is taken.

In fiscal 1989, the deficit would be reduced by nearly $46 billion.

Although many decisions on the specific cutbacks would be left for the upcoming legislation, the negotiators agreed not to reduce cost-of-living increases for Social Security recipients or other pensioners.