Some of the nationally known friends that Rep. Frank Thompson Jr. (D-N.J.) has acquired and helped during his 25 years in Congress are returning the favors by setting up a legal defense fund for him.

Letters went out this week to 5,000 leaders in the arts, labor, civil rights and feminist movements seeking money to help Thompson defend himself in the government's Abscam "sting" investigation.

The organizing committee behind the appeal includes AFL-CIO President Lane Kirkland; NAACP President Benjamin Hooks; producer Roger L.Stevens; Eleanor Smeal, chair of the National Organization for Women, and Msgr. George Higgins of the National Conference of Catholic Bishops. a

"Tompy has always stood with us," their letter said, citing his "public service of the highest order" and urging donations to defray his legal expenses in the Abscam affair.

In addition, Kirkland and AFL-CIO secretary-treasurer Thomas Donahue have sent a letter to each of the labor federation's 106 affiliated unions, seeking personal donations for Thompson.

"We are committed to the success of this effort," Kirkland and Donahue wrote.

Thompson, 61, elected in 1954, is a key figure on Capitol Hill labor policy. He is the second-ranking Democrat on the House Education and Labor Committee and chairs its labor-management relations subcommittee. He also is chairman of the House Administration Committee.

Thompson and seven other members of Congress are targets of an FBI investigation involving allegations of payoffs in return for promises to help a mythical Arab sheik with legislative favors. The FBI called the operation Abscam.

Government sources have claimed that Thompson and a companion accepted $50,000 in cash during an encounter with undercover FBI agents in a Washington house last fall.

The effort by Thompson's friends to raise defense money apparently is unique among the congressional targets of Abscam. None of the others is being helped similarly.

A spokesman for Sen. Harrison A. Williams Jr. (D-N.J.), however, said that some of his political allies are in the process of establishing a fund, but have not completed their work.

Rep. Richard Kelly (R-Fla.), another of the FBI's targets, has received a number of unsolicited contributions, all in small bills totaling less than $100, according to an aide. He said no consideration has been given to formation of a defense fund.

The Save Tompy campaign, calling itself the Citizens Committee for Frank Thompson's Defense, is being directed by Victor Kamber, an assistant to the president of the building and construction trades department of the AFL-CIO.

Kamber said that a number of Thompson's friends had approached the congressman about raising money in his behalf after the Abscam story broke in February. He said Thompson accepted their offer, since his legal fees are expected to be substantial.

The 5,000 letters were mailed to persons in activities Thompson has championed in Congress -- labor, human rights and the arts. Kamber said that no fund-raising goal has been set.

Thompson is a trustee of the John F. Kennedy Center for the Performing Arts here. Producer Stevens, a consigner of the fund-raising letter, is chairman of the trustees.

One of Thompson's chief aides on Capitol Hill, Hugh G. Duffy, counsel for the Administration Committee, denied reports that he and other congressional employes had solicited money for Thompson from their offices.

Sources had indicated that assistants on Thompson's two committees played an active role in seeking defense money from the congressman's labor-movement sympathizers.

But Duffy said his role and that of other staffers involved no more than responding to calls from Thompson's friends who wanted to help him.

"Before the Kirkland letter went out, people had not idea what they could do. There are dozens and dozens of people who want to help and we have had calls from the," he said.

Duffy said that all such calls offering assistance have been channeled to the citizen-committee trustee, lawyer Thomas R. Jolly, a former education and labor staff assistant.