At a White House ceremony crowded with labor, civil rights and other liberal political leaders, President Carter yesterday signed the Humphrey-Hawkins "full employment" legislation, but used the occasion to focus attention as much on inflation as on joblessness.

"I must warn you that our fight against inflation must succeed if we are to attain our unemployment goals," the president told the legislation's most avid supporters in the East Room of the White House. "Success in fighting inflation is critical to success in fighting unemployment."

Carter's use of the signing ceremony to drum up support for the administration's new anti-inflation plan was a final symbol of the changes the legislation went through before finally gaining the president's support late last year.

Named after its principal sponsors, the late senator Hubert H. Humphrey (D-Minn.) and Rep. Augusta F. Hawkins (D-Calif.), the bill, in its original form, would have committed the federal government to reducing overall unemployment to 3 percent, required massivenew job programs to reach that goal and instituted comprehensive national economic planning.

During his campaign for president, Carter endorsed the "principles" of the legislation but not its specifics. Once in office, he resisted giving the early versions of the measure his backing, fearing that the cost of the job programs it mandated would push inflation beyond control.

The bill Carter signed yesterday, the result of intensive negotiations between its supporters and the White House, was a shadow of the original version. It requires the president annually to set forth goals for employment, unemployment, production and inflation for the succeeding five years, and it sets as a national goal the reduction of the overall unemployment rate to 4 percent by 1983.

It also requires the president each year to recommend policies aimed at reaching these goals, but it requires the federal government to do nothing should the effort fall short.

At the same ceremony, Carter signed a four-year extension of the Comprehensive Employment and Training Act, designed to increase emphasis on training rather than public service jobs and cut down on fraud and abuse in CETA job programs.

Speaking of the latter provisions, the president said that those who abuse CETA . . . the main federal job-creation program . . . to enrich themselves "deserve condemnation." There have been localized but frequent reports of fraud in the CETA program, and Carter pledged that "we intend to see such abuses are ended."

The president was flanked during the ceremony by Hawkins and Sen. Muriel Humphrey (D-Minn.), the late senator's widow, who was appointed to serve out his term.

Around Carter, too, were many of the black congressional leaders who pushed the president for more active support of the "full employment" legislation during the closing weeks of the last Congress, by which time inflation had become the overriding concern inside the White House.

The president said the CETA extension will give the federal government the tools needed to reach some of the unemployment goals. Of the criticism that the Humphrey-Hawkins legislation had been watered down almost to meaninglessness, he added:

"This bill is filled with great and important substance . . . If it wasn't, the struggle [to enact it] wouldn't have been so hard."