Flanked by nearly two dozen civil rights and political leaders, President Carter yesterday announced a streamlined federal effort to crack down on employment discrimination.

"I believe this is the single most important development in civil rights in the last decade," the president told several hundred persons, about one-fourth of them black, assembled in the East Room of the White House. He called the gathering "truly an historic occassion."

Unless Congress votes against Carter's plan within 60 working days, the efforts to eliminate job discrimination now scattered among 15 federal agencies will be consolidated into three by July 1979.

A newly expanded and restructured Equal Employment Opportunities Commission would be the main enforcement agency, with the Labor Department monitoring compliance from federal contractors, and the Justice Department retaining the power to use state and local governmentS.

In a message to Congress, Carter promised to "aggresively enforce our civil rights laws," and said the slow buildup of more than 40 equal employment opportunity laws since the first one in 1940 has brought ". . . confusion and ineffective enforcement . . . regulatory duplication and needless expense for employers."

He singled out the Civil Service Commission for comment, saying it has been "lethargic in enforcing fair employment requirements within the federal government."

Carter's plan would transfer that power from the Civil Service Commission to the EEOC.

In a report issued last year, the EEOC said that, because of continuing job discrimination by employers, it might take almost a half-century more for blacks to attain their fair share of jobs at all levels in the nation's work force.

Carter asked last month for a $13.9 million increase in EEOC funding for fiscal 1978, bringing the total to $91 million, and an increase of $19 million for fiscal 1979, bringing that total to $110 million.

Office of Management and Budget officials said those totals would increase by $10 million move once the budgets from the agencies whose functions EEOC is taking over are included.