Ronald Reagan returned to the campaign trial today with more criticisms of President Carter and a pledge to "defend the integrity of the Social Security system."

In a speech to an audience of older Americans on a day designated here as "Super Senior Sunday," Reagan rapped Carter for proposing changes that were never made in the way Social Security payments are adjusted for inflation. He also said a study authorized by Carter had called for an income tax on Social Security benefits that Reagan termed "double taxation." i

Reagan was referring to a September 1979 recommedation by the Social Security Advisory Council, a 13-member body that meets once every four years to consider changes in the system. The council proposed making half of Social Security benefits subject to U.S. income tax for the first time since the system was created in 1935.

As the Republican presidential nominee spoke and the Glenn Miller-Jimmy Henderson Orchestra played "In the Mood," representatives of the Carter-Mondale Committee passed through the crowd handing out leaflets that accused Reagan of wanting a voluntary Social Security system.

The leaflets quoted various Reagan statement of several years ago, including one he made in 1966. Reagan said then that the government should explore a proposal that would allow people to use a voluntary retirement plan if they didn't want to participate in Social Security.

Reagan has made no such statement in this campaign. In fact, he has consistently promised -- as he did today -- to make sure that no changes will be made in Social Security benefits.

Reagan also has said that as president he would appoint a commission to study ways of improving the fiscal soundness of the Social Security trust fund. He did not repeat that statement in his speech here from the steps of the Philadelphia Museum of Art.

"As president, I will defend the integrity of the Social Security system, the foundation of the economic life of millions of Americans," Reagan said. "That system will be strong and reliable and protected under a Reagan administration."

Before giving what some have called his "peer group speech," the 69-year-old Reagan visited an older Americans' fair and dropped in on information booths promoting citizenship and the Foster Grandparents program.

Later, just before Reagan was to speak to Republican workers at St. Joseph's fieldhouse, two young demonstrators unfurled banners that said, "Christ is betrayed by nuclear weapons." The banners were ripped from their hands by members of the audience and the youths, who did not resist, were escorted from the arena by Secret Service agents.

"Is there anyone in the world who does not believe that Christ is betrayed any time we find ourselves embroiled in that greatest of man's stupidity -- or against our fellow man?" Reagan declared to the cheers of 2,000 partisans.

He then went on to say, to more cheers, that he had opposed the SALT II treaty because it increased armaments, and that as president he would try to negotiate a treaty with the Soviets "to reduce the nuclear weapons of both sides to where we represent no threat to each other."

It was a purposely light day for Reagan, who came off two days of rest at his rented rural Virginia estate for a week that will be devoted to economics and ethnicity.

On Monday, Reagan will breakfast with Cardinal John Krol. Later in the day he will meet with Polish-Americans in Chicago. On tuesday, also in Chicago, he will issue a long-promised and long-delayed economics program in a speech to the International Business Council.

One Reagan aide, asked what the candidate hopes to accomplish this week, replied succinctly -- "Give a good economics speech and stay out of trouble."

Reagan strategists hope that a trouble-free campaign week will put an end to press emphasis on a variety of oral slips Reagan made in late August and early September.

Reagan is being kept as isolated from the press as possible. He has given no news conference since a troublesome one Aug. 25 that focused on the Taiwan issue. No news conference is scheduled this week.

Reagan spokesman Lyn Nofziger said that his only commitment would be to see that "Reagan has press conferences more often than Jimmy Carter."