President Reagan yesterday dismissed Walter F. Mondale's deficit-reduction plan as "nothing new," and his administration joined congressional Republicans in a coordinated attack on the proposal as an "economic disaster" that would increase tax burdens for average Americans.

Democrats strongly praised Mondale for detailing his budget and tax plan and criticized Reagan for not doing the same. Some reacted cautiously to Mondale's plan to raise taxes, given the conventional wisdom that candidates should not seek painful tax increases in an election year. Mondale is gambling that candor will outweigh that.

Reagan, questioned by reporters at a White House picture-taking session, said, "This is nothing new. He told us several weeks ago he was going to raise the people's taxes, and now he's repeating it."

Asked if he would offer specifics about his deficit-reduction plans before the election Nov. 6, Reagan referred reporters to his past budget cuts and, sounding annoyed, cut off their questions.

"Now this is a photo opportunity, and I'm going to stay with the business at hand," he said.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said, "Today, we saw the contrast between the two nominees for president. Mr. Mondale gave us a budget. Mr. Reagan gave us a photo opportunity."

Republicans primarily criticized the tax aspects of Mondale's plan, not the spending side.

"Walter Mondale has proved again that the only way he knows how to cut budget deficits is to increase massively the deficits in the pocketbooks of average Americans," Reagan-Bush campaign consultant Lyn Nofziger said in a written statement.

White House spokesman Larry Speakes said that Mondale "would place a burden on the American taxpayer that would stop the economy cold in its tracks." He said Mondale's plan would result in higher inflation and interest rates and would "put the highest taxes in peacetime history on the backs of the American people by increasing the tax burden to nearly 21 percent of GNP gross national product ."

Acknowledging that Mondale's plan would produce a lower deficit by 1989 than Reagan is forecasting, Speakes refused to provide details of Reagan's deficit-reduction plans, which he said will come after the election, if Reagan wins. Speakes said Reagan remained committed to characterizing a tax increase as a "last resort" if budget-cutting fails to narrow the deficit significantly.

Treasury Secretary Donald T. Regan said the Mondale plan would cost a minimum of $1,000 in higher annual taxes for the average family by 1989.

House Minority Leader Robert H. Michel (R-Ill.) said that "there's enough fiscal fertilizer" in Mondale's budget "to send the deficit shooting up like Jack's beanstalk." Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) added: "When you take it piece by piece, there's something there to offend everyone."

Vice President Bush, campaigning in Raleigh, N.C., said Mondale had underestimated the cost of primary-campaign promises to increase spending.

Sen. John Glenn (D-Ohio), a Mondale rival in the Democratic primaries, said that, while he did "not agree with every element" of Mondale's plan, "there can be no dispute" that "it stands in stark contrast to what President Reagan is offering."

Reagan "might think this is a good way to run for reelection," Glenn said, "but he can't hide from the $200 billion deficits that he's created."

Senate Minority Leader Robert C. Byrd (D-W.Va.) said Mondale "demonstrated a lot of courage, backbone and guts" in his budget and tax proposal. Asked if Democrats would distance themselves from Mondale because of the new taxes, Byrd said, "I think we'll have to wait and see."

Rep. Richard A. Gephardt (D-Mo.), a leading advocate of tax simplification, said Mondale had offered "a realistic and constructive approach" and had been "open and honest with the American people."

House Budget Committee Chairman James R. Jones (D-Okla.) said Mondale "is to be commended for putting a specific deficit-reduction program on the table."

Some Democrats have expressed unease about running on a tax increase. Sen. Walter D. Huddleston (D-Ky.) said last week, "My personal opinion is that Mondale made a mistake in raising the tax issue." Huddleston said he would not campaign on the tax issue.

James A. Johnson, Mondale's campaign chairman, said Mondale has no fear of asking voters for a tax increase.

"We sense that dealing with the problem head-on is an enormous advantage," he said, "and to deal with the tax increases in a way that's fair to the average American family is, after all, what Democrats stand for."