President Carter today assailed a Republican tax-cut plan supported by Ronald Reagan as "sugar-coated poison" for all but the rich and promised that he will soon unveil his own economic recovery plan that will put "millions and millions" of Americans back to work. r

With providing any details, the president told the annual conference that his plan will modernize American industry, create jobs and hold down inflation at the same time.

White House officials said the administration proposal would be unveiled around Labor Day, the traditional start of the fall presidential election campaign, and that it will "contrast greatly" with the GOP call for a substantial federal income-tax cut to stimulate the economy.

Carter campaign strategists plan to make this contrast a major issue in the fall campaign, and the president warmed up for that task today with his harshest criticism to date of the Republican proposal.

Labeling the proposal as possibly "the most inflationary piece of legislation ever introduced and considered seriously by the United States Congress," Carter said it "offers rebates to the rich, and fierce inflation and deprivation for other Americans who are particularly vulnerable."

"It substitutes a fantasy of instant gratification instead of a realistic vision of a better future," he said. "It's even worse than a free lunch. It's sugar-coated poison."

The target of this presidential assault was the Kemp-Roth tax bill, which would reduce federal income taxes by 30 percent over three years. It has been endorsed by GOP presidential nominee Reagan, and its principle were adopted in the party platform by the Republican National Convention in Detroit.

Carter aides now refer to the measure as the "Reagan-Kemp-Roth" bill as they plot how to saddle the Republican candidate with what they believe will be perceived by November as a wildly inflationary proposal.

In his address to about 2,000 members of the Urban League the day after Reagan spoke to the group, Carter had to choose his words carefully. His speech to the national civil rights organization was considered official business and was paid for by the government.

As a result, the president was precluded from mentioning Reagan or the Republicans by name. But when he said that the Kemp-Roth proposal has been "endorsed by major political candidates" and enjoys the backing of "the same [people] that have opposed every form of social progress of the past generation," there was no doubt about his targets.

Carter also belittled other Republican promises.

"The same people who are pushing this tax also promise massive increases in the defense budget and they also promise to balance the budget," the president said. "Whom are they trying to fool?"

He said a study conducted by the administration showed that such a program of tax cuts, a "moderate" increase in defense spending and the retention of existing Social Security benefits would mean that "every other agency and department and program In the federal government would have to be eliminated 100 percent."

White House officials said later that this study, conducted by the Office of Management and Budget, is expected to be made in a few days.

Carter coupled his attack on Republican economic policies with a reminder to the largely black organization of the power of the nation's chief executive to appoint federal judges, another major theme he is expected to use in a fall campaign against Reagan.

Renewing what he called his "permanent pledge" to seek "equal rights, equal opportunity and equal dignity" for all Americans, the president said of the large number of blacks and other minorities he has appointed to the federal judiciary:

"Remember that these federal judges serve for life. Their influence extends far beyond any single presidential term . . . Don't forget that these judges will be interpreting your rights, the rights of your children, the rights of your children's children on into the next century."

With the economy continuing to slump, the president has come under increasing pressure from such groups as the Urban League and the nation's mayors to reduce unemployment. Carter has insisted that inflation remains the nation's number one economic problem. Without retreating from that position, his promise today to come up with a "carefully designed" economic recovery program appeared aimed at calming the increasingly restless elements in this own party just a few days before the opening here Monday of the Democratic National Convention.

The president said the program will not involve "massive programs that hide inflationary time bombs," but that it will "put people to work building the facilities that we need to conserve more energy, change the way we use energy and produce more energy."

Carter received a warm reception from the Urban League audience, but the speech was interrupted four times by hecklers from the Communist Workers Party, one of whom called him a "liar and murderer." The three women and one man, who stood and shouted insults at the president at apparently carefully timed intervals, were hauled from the meeting site by police.

Before returning to Washington, the president visited briefly with Vernon E. Jordan Jr., the president of the Urban League, who is recovering in a New York hospital from gunshot wounds he suffered in an attack by an unknown assailant in May. Carter said Jordan is "in very good spirits and he's recovering well."