President Reagan yesterday bluntly warned conservative Republicans who threaten to block an $8.4 billion authorization bill for the International Monetary Fund that, if they succeed, the consequences could be "an economic nightmare that could plague generations to come."

In an address to the opening session here of the IMF and the World Bank, Reagan pushed hard for passage of the legislation supporting an institution that he described as "the linchpin" of the entire international financial system.

"Let me make something very plain," Reagan told an audience composed of the world's top bankers and financial officials at the Sheraton Washington Hotel. "I have an unbreakable commitment to increased funding for the IMF."

American failure to participate in a proposed 48 percent expansion of the IMF's capital would result in a withdrawal by other major nations, and the resultant "nightmare" would disrupt the world's trade and financial systems, he said.

Reagan called on Democrats and Republicans alike "to lay aside their differences, and to get on with the task in a spirit of true bipartisanship."

But his plea did not go down well with Democrats on Capitol Hill, who have overwhelmingly supported the bill while the Republicans have been split down the middle.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said Reagan's speech would not satisfy Democrats who "saved his bill" and then were subjected to "the most vicious political letter I have ever seen" from the Republican Party.

He was referring to charges by Rep. Phil Gramm (R-Tex.) that supporters of the bill are helping communist countries.

Rep. David R. Obey (D-Wis.) and other Democrats have demanded that Reagan write a letter directly disavowing the Gramm charge. Without it, some Democrats worry that the Gramm charge will work against them in the elections next year.

Reagan's grim assessment of what would happen if Congress fails to pass the legislation was echoed in a separate address by IMF Managing Director Jacques de Larosiere, who said failure would quickly "cripple this institution," with "incalculable consequences for economic and financial stability worldwide."

The president, in one of his most optimistic recent assessments of the American economy, also said that economic growth is gaining momentum, not only cutting the U.S. budget deficit more than was expected but also acting as a tonic boosting worldwide activity.

Reagan pledged that the United States would cooperate with other powers to produce a lasting, global expansion. At the same time, he restated his Williamsburg summit pledge to resist protectionism, conceding that it is easier to adopt a free-trade policy than to carry it out.

But, in implicit recognition of demands made during the IMF/Bank sessions for a reduction of the U.S. budget deficit, Reagan took a strong position against any effort to cut the flow of red ink by raising taxes:

"We will continue to work for greater restraint in federal spending. But we will not risk sabotaging our economic expansion in a shortsighted attempt to reduce deficits by raising taxes."

He said that the deficit "is caused in part by our determination to provide the military strength and political security to ensure peace in the world."

But he said that defense spending cannot he cut until arms reduction agreements are reached with the Soviet Union.

In pressuring Congress to approve the $8.4 billion for the IMF, Reagan stressed a point that the administration has not been able to drive home before: the money does not come out of the budget, but as a loan, "it will be returned to the Treasury with interest."

Reagan said the United States also strongly supports the World Bank, and made a brief appeal for funds to continue operations of the bank's concessional-aid arm, the International Development Agency.

But he carefully avoided making a specific commitment to the IDA program, scheduled to begin in mid-1984.

On that issue, World Bank President A.W. (Tom) Clausen made an impassioned plea in his annual address to the joint meetings, saying that the United States and other major donors who have failed to agree must quickly make a decision "at the political level" or risk "a hiatus" in IDA operations.