Rep. Ralph H. Metcalfe (D-Ill.), a leading black legislator whose 6th Congressional District covers much of Chicago's South Side, and a former Olympic athlete, died yesterday in Chicago after an apparent heart attack.

He was stricken at his home in the South Side and was pronounced dead at Michael Reese Hospital. Rich Weston, his public relations director, said Mr. Metcalfe, 68, had suffered a heart attack in 1970, and also was a diabetic.

The former Olympic star was first elected to Congress in 1970, and was reelected to each succeeding term despite a fallout with his old and close friend, the late Chicago mayor Richard J. Daley, over police treatment of blacks.

Mr. Metcalfe was up for reelection this year but had only token opposition from a Republican candidate, Weston said. The congressman had gone to Chicago for the weekend, as was his wont and was expected back yesterday for the final sessions on Capitol Hill.

President Carter described Mr. Metcalfe yesterday as a leader "uncompromising in the pursuit of excellence."

"He stood always for equity. His political energy was invested always on behalf of his constituents, and his unexpected passing deprives his community, his colleagues and the nation of a staunch and honest leader," he said.

Mr. Metcalfe came up through the political ranks, starting in 1952 when he was elected to the Cook County Democratic Committee from the city's 3rd Ward, a seat he held until his death.

Three years later, he was elected to the Chicago City Council as an alderman from the 3rd Ward, and in 1969 he was Daley's personal choice to serve as president pro tem of the council.

He was considered one of Daley's most trusted black lieutenants and easily won his congressional race in 1970, when he took over the seat of the late representative William Dawson. He went on to become a founding member of the Congressional Black Caucus.

By 1972, however, Mr. Metcalfe's relationship with Daley had shifted. He became highly vocal as he joined some of Chicago's black activists in protesting what they considered to be "selective police brutality" in the treatment of blacks.

"I know the political realities of what I'm doing. . .I'm willing to pay whatever political consequences I have to, but frankly I don't think there will be any," he said, adding:

"In the Black Caucus we have decided to put the interests of black people first - above all else - and that means even going against our party or our political leaders if black interests don't coincide with their positions."

He successfully fought the Daley machine although Daley stripped him of patronage.

In Congress, Mr. Metcalfe was a member of the Interstate and Foreign Commerce Committee, sitting on its Transportation and Commerce and Consumer Protection and Finance subcommittees.

He was chairman of the Panama Canal Subcommittee of the Merchant Marine and Fisheries Committee and favored the Panama Canal treaties. He also was on the Merchant Marine and Oceanography subcommittee and sat on the Post Office and Civil Service Committee.

He was a member of the Democratic Steering and Policy Committee, vice chairman of the Democratic Study Group and belonged to Members of Congress for Peace Through Law and the Northeast-Midwest Economic Advancement Coalition.

Mr. Metcalfe was born in Atlantic and grew up in Chicago.He graduated from Marquette University in 1936. Before college, he was national interscholastic champion in sprinting. At Marquette, he was a national collegiate champion sprinter.

He participated in the 1932 Olympic Games in Los Angeles where he won silver and bronze medals in the 100 and 200-meter sprints. He also was in the 1936 Olympics in Berlin, winning a gold medal on the 400-meter relay team and a silver medal for finishing second to Jesse Owens in the 100 meters.

Mr. Metcalfe had been a member of the President's Commission on Olympic Sports since 1975. Earlier, he had been the first black to be named to the Illinois State Athletic Commission (1949-52).

During World War II, he served first as a USO club director at Fort McClellan, Ala., and then as an Army-director of physical education. He received the Legion of Merit.

He founded the Ralph H. Metcalfe Youth Foundation, which was devoted to athletic, health and educational programs not only for young people but also for needy families.

Mr. Metcalfe had been active in the NAACP, the AMVETS and the American Legion.

He is survived by his wife, Madalynne Fay Young Metcalfe, of the homes in Chicago and Chevy Chase; a son, Ralph H. Jr., his mother, Mayme Attaway Metcalfe, and his stepmother, Effie Mae Metcalfe, all of Chicago, and one grandchild.