President Reagan yesterday deplored the increase in the nation's official unemployment rate to its highest post-World War II levels, but he again attempted to diminish the significance of the numbers by noting they had been adjusted for seasonal variations in employment.

Reagan said both unemployment, which soared to 9.4 percent last month, and high interest rates will fall if Congress will adopt the budget now before it--proposals the White House and Senate Republicans agreed on late Wednesday after the Senate Budget Committee unanimously rejected the budget Reagan had sent to Capitol Hill in February.

"Something must be done and can be done about unemployment," he said in his weekly, five-minute radio address. "If Congress will get off the dime and adopt the deficit-reducing budget it now has before it, interest rates will come down when it does, and so will unemployment. This is no time for politics as usual. There are too many people hurting."

Reagan barraged listeners with statistics in a two-fisted assault on Democrats and the press for suggesting that he has cut social spending and will have to reduce Social Security payments.

"Those who rush to face the TV cameras or get their names in newsprint by frightening our Social Security recipients should be ashamed of themselves," he said. "Let me repeat what I've said before. I will protect the benefits of Social Security recipients now and in the future."

He shed no light, however, on how he will be able to carry out that pledge, and at the same time achieve the $40 billion in Social Security savings over the next three years that is called for in the budget compromise he worked out with Senate Republicans. Reagan has said he opposes using general revenue funds to replenish the Social Security trust fund, which would appear to leave the options of reducing future cost-of-living increases for beneficiaries, increasing the payroll taxes of those still working, or restructuring the system in some way.

Reagan assured pensioners that they would get their 7.4 percent cost-of-living increase on July 1 as scheduled. But he made no such explicit promise about granting full cost-of-living increases in future years.

Last week White House advisers were deflecting that political "hot potato" by asserting that it was Democrats who had suggested cutting back on cost-of-living increases in the abortive bipartisan budget negotiations last month. The presidential aides repeated that they intend to await the recommendations of the bipartisan 15-member Presidential Commission on Social Security Reform before deciding what to do about Social Security. The commission is not to issue its report until after the November elections.

In the Democratic response, broadcast on radio an hour after Reagan's mid-day speech, Sen. Christopher J. Dodd (Conn.) said it takes only "simple arithmetic" to determine that the savings in Social Security that Reagan is proposing would mean lowering benefits by an average of $1,000 for 39 million recipients.

Dodd also accused Reagan of delivering a "campaign stump speech" in an effort to escape blame for the slide in the economy.

"What our president argues is that all of our economic troubles should be laid at the doorstep of the Democratic Party, the news media, business, labor, consumer groups--in fact, everyone except his own administration and the Republican members of the Senate Budget Committee.

"Now you and I know that argument is absolutely ridiculous."

Reagan, in his speech, said the unemployment rate reported by the Bureau of Labor Statistics on Friday was, "of course, bad news." But, continuing an argument he made last month to an eighth-grade civics class in suburban Chicago, Reagan faulted the press for reporting only the seasonally adjusted numbers for unemployment and not the raw figures, which show more people working and fewer unemployed.

Basic unemployment figures, published by the bureau each month, are altered to filter out the fairly regular seasonal fluctuations in employment due to such events as changes in weather, reduced or expanded production, harvests and major holidays.

The bureau notes that the labor force, for example, increases by a large number each June when schools close and young people enter the labor market.

In Chicago, Reagan told the schoolchildren that "the statisticians in Washington have funny ways of counting." Yesterday Reagan was not as chiding, noting that next month, when 750,000 persons are out of school, the seasonally adjusted figures will probably look better than the unadjusted.

But, the president said, "I'm not sure we live in a seasonally adjusted world."

In his continuing campaign to counter perceptions that he is unfair to the poor, Reagan again asserted that "there've been no cuts in the budgets."

"There have only been smaller increases than some of our big spenders would have preferred, coupled with what we've done in our tax cuts to allow you to keep more of what you earn."

That is an assertion contradicted by his own budget document, which shows, among other cuts, an actual reduction of $3.805 billion in the current fiscal year in federal spending for training, employment and labor services and a cut of $3.695 billion for social services programs.