President Reagan lifted the embargo on grain sales to the Soviet Union yesterday and said he is certain the Soviets and others around the world will not mistakenly think he has weakened his stand against Soviet aggression.

"We will react strongly to acts of aggression wherever they take place," Reagan said in his statement announcing that he was wiping out the most serious action the Carter administration took to punish Moscow for its invasion of Afghanistan.

Reagan entered office determined to establish a tough and consistent posture from which to deal with the Soviet Union. The lifting of the embargo, an action Moscow has sought, was Reagan's first major act bearing directly on the U.S.-Soviet relationship.

It stands in apparent contrast to the president's anti-Soviet rhetoric -- including his Jan. 29 charge that Soviet leaders will lie, cheat and commit any crime to achieve their goals -- as well as such minor harassment of Moscow as depriving Ambassador Anatoily F. Dobrynin of his unique State Department parking privilege and abruptly refusing a visa extension to Georgy Arbatov, Moscow's chief America-watcher.

One day earlier, a White House official told reporters that one reason for the administration's decision to sell sophisticated AWACS radar planes to Saudi Arabia was the increasing Soviet threat in the Persian Gulf region.

An administration official said the decision to lift the grain embargo was made possible in part by lessened tension in the world, including in Poland, where fears of a Soviet invasion have lessened.

The official, who spoke to reporters on the understanding that he not be identified, said he sees no contridiction or inconsistency in the two actions.

Senate Minority Leader Robert C. Byrd (D-W.Va.) said the decision "gives our nation an image of softness and vacillation."

Byrd added, "What we are being told now, basically, is that the Soviets should be rewarded for being good for a few days. Lifting the embargo in no way enchances the security of Poland, and makes the United States appear weak and lacking in resolve."

Since taking office, Reagan has been caught between his campaign pledge to eliminate the embargo -- a pledge that helped him win farmers' votes last November -- and warnings from Secretary of State Alexander M. Haig Jr. and others that lifting the embargo would send the wrong signal to the Soviets.

From Jan. 4, 1980, when President Carter imposed the embargo in response to the Soviet invasion of Afganistan, Reagan has been critical of it on the grounds that it asks American farmers to bear an unfair burden.

At one point during the campaign, Reagan suggested that a more appropriate and effective way to punish Moscow might have been to blockade Cuba.

He has said repeatedly that the embargo did not hurt the Soviet Union, which was able to buy the grain it needs from other nations.

Reagan returned to these themes in yesterday's statement, which was read for reporters and the television cameras by deputy White House press secretary Larry Speakes.

Reagan stressed that his action was taken in fulfillment of his campaign promise, and called the embargo an "ineffective national policy." He said that lifting the embargo has been under constant review.

"In the first few weeks of my presidency, I decided that an immediate lifting of the sales limitation could be misinterpreted by the Soviet Union," Reagan said. "I therefore felt that my decision should be made only when it was clear that the Soviets and other nations would not mistakenly think it indicated a weakening of our position.

"I have determined that our position now cannot be mistaken: The United States, along with the vast majority of nations, has condemned and remains opposed to the Soviet occupation of Afghanistan and other aggressive acts around the world. We will react strongly to acts of aggression wherever they take place. There will never be a weaking of this resolve."

Neither the unnamable administration offical nor Reagan, who made the decision on his first day back in the Oval Office since he was shot March 30, explained what action or actions had led him to determine that conditions 30, explained what action or actions had led him to determine that conditions now permitted him to lift the embargo.

The absence of a Soviet crackdown against the Polish labor unions was only a partial factor in the decision, this official said, but he would not list the other factors.

"I would not peg the president's decision to lift the embargo to any specific action," the official said, adding, "This is a principled decision and we insist that you accept it as such."

He rejected the suggestion that the Soviets were being rewarded.

Several diplomats who deal with the Soviet Union let it be known that they believe that lifting the embargo will make it harder to rally support for criticisms of the Afghanistan situation and other Soviet actions.

They also said that the decision will strengthen the hand of those Soviet leaders who argue that if Moscow stands firm the United States will either change administrations or change policies and cave in.

Even before the embargo officially ended at 4 p.m., Soviet officials were in the Agriculture Department building here discussing grain purchases, Agriculture Secretary John R. Block said.

If the Soviets appear eager to buy, the Americans appear no less eager to sell. By happy coincidence, USDA officials discovered that about 6 million metric tons of corn that hadn't been noticed before are availabe for sale. In addition, officials said, there is plenty of wheat, and a handsome new crop is on the horizon.

"I'm happy beyond comprehension. It has been a long 100 days," said Block, who had pushed for an end to the embargo.