President Carter, responding to Ronald Reagan's nationally televised foreign policy speech Sunday night, described his Republican opponent today as "extraordinarily naive" about nuclear arms and said Reagan seemingly "does not understand the serious consequences of what he's proposing."

Determined to keep the so-called "war and peace" issue in the forefront of the presidential campaign, Carter read a prepared statement to reporters at the White House this morning before flying to this economically depressed area of eastern Ohio where far more politically discomfiting questions about unemployment and inflation awaited him.

In the process, the president took certain liberties with the content of the Reagan speech, in which the GOP nominee reiterated his opposition to the pending strategic arms limitation treaty (SALT II) with the Soviet Union but said he would seek to negotiate a SALT III with the Soviets that would result in real arms reductions.

"After reading his speech last night, I'm concerned that he does not understand the serious consequences of what he's proposing," Carter said as he described the Reagan proposal as consisting of:

"First, throw the existing nuclear arms limitation treaty in the wastebasket. Second, threaten the Soviet Union with a nuclear arms race. Third, launch a quest for so-called nuclear superiority. Finally, make the naive assumption that the Soviet response to all these steps will be to agree to new concessions and reductions in their nuclear arsenal.

"Can anyone seriously believe that this would actually happen?"

The president answered his rhetorical question, calling Reagan's proposals "extrordinarily naive" and predicting they would be "a devasting, and perhaps fatal blow to the long-term process of nuclear arms control."

Carter's prepared statement on nuclear arms control was read to reporters on the south lawn of the White House and timed for the television networks' morning news programs. Coming just as the president was to take off for Beaver Falls, Pa., and Youngstown, it neatly symbolized the strengths and vulnerabilities of the two major party presidential candidates.

For a Republican, Reagan has spent an inordinate amount of time in the coal and steel producing areas of western Pennsylvania and eastern Ohio that the president visited today; Reagan plainly hopes to exploit the area's double-digit inflation and unemployment to cut into the normally Democratic vote.

For their part, Carter and his strategists are convinced the "war and peace" issue offers the surest path to the president's reelection. They are determined to keep hammering on that theme, as Carter did today, particularly in the days leading up to next week's scheduled debate between the candidates.

In eastern Ohio today, however, the citizens of Youngstown were not ready to cooperate with that campaign strategy. Forty of them gathered in a television studio for a scaled-down version of the "town meetings" the president has been holding around the country, and again and again ther questions had to do with the economy.

In response, the president offered what has become his standard dwelling on the "inconveniences and the transient problems" of the present, reminding the audience that the country has been through more difficult times in the past.

To a womand who said the merchants of Youngstown seemed to have lost faith in the country's economic system as Christmas approaches, Carter said, "We're well on the way to recovery. I think we'll have a good Christmas."

The few foreign policy questions asked during the hour-long session centered on the hostages in Iran. The president replied by giving his most explicit pledges to date of cooperation with the Iranian government in return for release of the hostages.

From Youngstown, Carter flew later today to New York for a whirlwind series of campaign stops. Joined by Sen. Edward M. Kennedy (D-Mass.) and former heavyweight boxing champion Muhammad Ali, he first addressed an audience at a black church in Brooklyn.

Later, the president spoke to the Asian Pacific American Committee dinner, addressed a group of local Jewish leaders and, joined by Kennedy, ended the day with a speech at a Democratic Party fund-raising dinner in Manhattan.