President Reagan's assertion that the Soviet Union is ahead in the arms race and that the United States must catch up before negotiating a freeze in nuclear weapons provoked sharp criticism from pro-freeze members of Congress yesterday.

Responding to the president's Wednesday night news conference, Sen. Edward M. Kennedy (D-Mass.) said at a news conference of his own that Reagan was wrong when he claimed "the Soviet Union does have a definite margin of superiority" in nuclear weapons.

"No one in authority, including President Reagan, would trade our deterrent for the Soviet forces," Kennedy said.

Asked later if the president would indeed swap U.S. nuclear arms for the Soviet forces he described as superior, White House deputy press secretary Larry Speakes replied, "That strikes me as highly unlikely."

Kennedy and Sen. Mark O. Hatfield (R-Ore.), cosponsors of a Senate resolution calling for a freeze on both superpowers' nuclear arsenals, called a news conference whose sole purpose was to dispute statements the president made at his news conference, and 17 television cameras showed up to record the event.

Kennedy and Hatfield suggested that the media's intense new interest in nuclear arms reflected deeply felt public concern. They said pressure would continue mounting to force the administration to do more to control nuclear weapons. "The people are ahead of the politicians," Kennedy said.

Reagan rejected the Kennedy-Hatfield approach to a nuclear freeze at his news conference, endorsing instead a rival resolution supported by a majority of senators. That says there is an "imbalance" in nuclear arms favoring the Soviets that must be eliminated before a freeze can take effect.

"The president said in effect that we have to build more nuclear bombs in order to reduce the number of nuclear bombs," Kennedy said yesterday. "This is voodoo arms control."

Hatfield asked why the president is willing "to let the Soviet Union develop a new generation of domestic missiles" by delaying a freeze, instead of trying to head off the next generation by trying to negotiate a halt in the arms race at present levels.

Kennedy said it was "dangerously naive" to think "we can now build more while the Soviets will do nothing."

Rep. Edward J. Markey (D-Mass.), sponsor of a pro-freeze resolution in the House, said Reagan "does the country a great disservice" and "misstates the case" when he claims the Soviets have superiority. "I just wish the president would stop bad-mouthing the nation's defenses," Markey said.

Sens. John Glenn (D-Ohio) and J. James Exon (D-Neb.), both moderates not active in the freeze movement, also attacked Reagan for saying the United States is inferior to the Soviets.

Exon said he was "astonished" by Reagan's statement on Soviet superiority, "which I am sure will send most unsatisfactory shock waves everywhere, especially among our allies. To foster the belief that we are not essentially equal with the Soviets in overall nuclear strength is nonsense."

Other administration officials sought to take some of the edge off of the president's claim that the Soviets now enjoy "a definite margin of superiority," which is the strongest statement Reagan has made on the subject since he became president.

Appearing on the CBS Morning News, the director of the State Department's office of politico-military affairs, Richard R. Burt, said that Reagan's assertion was "something I think presidents and secretaries of defense and secretaries of state have been saying for several years now--that the Soviets have the momentum in the strategic arms competition, and we are worried about the trends."

In fact, not even Reagan's secretary of defense, Caspar W. Weinberger, has gone as far as Reagan did Wednesday night in ascribing a "definite margin of superiority" to the Soviets.

Secretary of State Alexander M. Haig Jr. has said "the United States is very, very strong and very, very capable, especially in the strategic area. Our systems are both more sophisticated and reliable and more technologically sound."

The chairman of the Joint Chiefs of Staff, Gen. David C. Jones, has said he would not swap the American defense establishment for the Soviet version. Gen. Lew Allen Jr., the Air Force chief of staff, said last year that "the U.S. Air Force which includes America's land-based missile force is the finest in the world . . . . I would not trade the United States Air Force for its Soviet counterpart."

Reagan's view that the Soviets are superior was endorsed yesterday by Sen. John G. Tower (R-Tex.), chairman of the Senate Armed Services Committee. Many hard-line critics of past administrations' defense policies also have said the Soviets have superiority.

Congressional critics also challenged Reagan's news conference statement that "the Soviets' great edge is one in which they could absorb our retaliatory blow and hit us again."

Kennedy said the United States would have more than 3,000 surviving warheads after a Soviet first strike that could "vaporize" Soviet society. Rep. Thomas J. Downey (D-N.Y.) said superior U.S. submarine forces actually would provide a more survivable retaliatory force than anything the Soviets have.

The United States has 9,000 warheads on intercontinental weapons that can strike the Soviets. The Soviet Union has 7,000 comparable warheads, but they are bigger and together have more explosive power than the U.S. arsenal. Picture1: Rep. Edward J. Markey...the president 'mistakes the case'