Secretary of State Alexander M. Haig Jr., seeking to emphasize that lifting the Soviet grain embargo does not mean a lessening of President Reagan's hard line in East-West relations, yesterday called the Soviet Union "the greatest danger to world peace."

Only the United States has "the pivotal strength" to block aggressive Soviet behavior, Haig said.

Haig addressed the American Society of Newspaper Editors less than an hour after Reagan ended the embargo. Although his speech was billed as a "philosophical" explanation of Reagan's foreign policy, Haig used it to try to underscore that the administration is not retreating from its confrontational style in dealing with Moscow.

His appearance also was notable because it came against a background of continuing speculation about whether Haig's recent, much-publicized scrapes with the senior White House staff and other administration officials over policy control have eroded his power and effectiveness and possibly even marked him as a man who is being eased out of office gradually.

That theme, although not a part of his speech text, was a palpable part of the two hours he spent with the editors. It was alluded to jocularly when Haig was introduced to the luncheon audience at the Washington Sherator Hotel, and even the secretary joked about it at the outset of his remarks: "It's not true that I eat Scott's Turf Builder with my Cheerios for breakfast every morning."

The same theme underaly several questions from the audience. In response, Haig patiently insisted that there are no serious differences between him and other administration officials such as Defense Secretary Caspar W. Weinberger and, on the question of his known opposition to lifting the grain embargo, he said: "It is my policy to fully support the presidency."

Asked about the concern of some Reagan loyalists that he has presidential ambitions of his own, Haig replied, "Would anyone in his right mind choose the office of secretary of state as the road to the White House?

"I am here to serve President Reagan," he added. "That is an office and duty I am proud to hold."

In other responses and in his prepared speech, Haig appeared to go to special lengths to put across the message that lifting the embargo should not be viewed by the Soviets or the rest of the world as signaling a softer line in dealing with Moscow.

The United States, he said, never will accept Soviet actions such as the invasion of Afghanistan, the event that prompted former president Carter to impose the grain embargo in early 1980.

Charging that "Moscow continues to support terrorism and war by proxy," Haig cited such U.S. policies as helping the government of El Salvador to resist leftist guerrillas as signs of the administration's reassertion of American power.

"Only the United States has the pivotal strength to convince the Soviets -- and their proxies -- that violence will not advance their cause," he said. "We have a right, indeed a duty, to insist that the Soviets support a peaceful international order, that they abide by treaties and that they respect reciprocity. A more constructive Soviet behavior in these areas will provide the basis for a more productive East-West dialogue."