Sen. Edward M. Kennedy (D-Mass.) assailed William H. Rehnquist yesterday as "too extreme to be chief justice" as the Senate Judiciary Committee began its confirmation hearings on President Reagan's nomination of Rehnquist to head the Supreme Court.

Kennedy staked out his uncompromising position in an unusual move at the outset of what promises to be a tense and perhaps bitter fight over the nomination. Several committee Republicans promptly affirmed their unqualified support of Rehnquist, presently an associate justice, and Sen. Orrin G. Hatch (R-Utah) said that he regarded a number of the complaints about the nomination as "somewhat ridiculous."

Rehnquist did little more than present himself at the witness table for a brief hello before Sen. Strom Thurmond (R-S.C.), the committee chairman, called the hearing to a halt at 7:30 p.m.

The session began late in the afternoon, delayed by a tardy flight of senators returning from funeral services in New York for the diplomat and politician W. Averell Harriman. Opening statements from the lawmakers took up virtually the entire proceeding.

Listing his objections to Rehnquist in rapid-fire order, Kennedy said that the justice's record since his appointment to the court by President Nixon in 1971 showed that Rehnquist had voted "against the individual 77 percent of the time in cases involving individual rights" and was the lone dissenter in 54 cases decided on the merits.

"Mainstream or too extreme, that is the question," Kennedy said. "By his own record of massive isolated dissent, Justice Rehnquist answers that question -- he is too extreme on race, too extreme on women's rights, too extreme on freedom of speech, too extreme on separation of church and state, too extreme to be chief justice."

Rehnquist listened quietly in a front row seat. He faces a prolonged round of questioning starting today.

The issues that critics have raised include a series of memos on racial issues Rehnquist wrote as a Supreme Court clerk in 1952; his role as a Republican activist in charge of controversial GOP "ballot security" programs in Phoenix in the early 1960s, and his truthfulness about those matters when they were first raised during the 1971 confirmation proceedings.

Hatch wryly suggested that "many of these alleged concerns" were too old to take seriously, predating even the Hula Hoop and even going all the way back to the days when Sen. Thurmond was still "a misguided Democrat."

The Utah Republican contended that emphasis on Rehnquist's lone dissents was also exaggerated. In the past 10 years, Justice John Paul Stevens has been a lone dissenter 51 times to Rehnquist's 40, Hatch said.

He added that Justices William J. Brennan Jr. and Thurgood Marshall "have been in dissent by themselves hundreds of times" over longer careers.

"Some of these assertions here today are somewhat ridiculous," Hatch said.

Not all the Democrats on the committee appeared to share Kennedy's views, at least at this stage of the proceedings.

Sen. Joseph R. Biden Jr. (D-Del.), the ranking minority member of the committee, confined himself to a lengthy statement on the importance of the hearings and of the need for the committee to satisfy itself on a variety of issues, including the state of Rehnquist's health. Biden gave no hint of how he intended to vote but he kept referring to the nominee as "chief justice."

Kennedy, as leader of the liberal and apparently outnumbered opposition, denounced Rehnquist for what Kennedy described as an "appalling record on race," including his authorship of a 1952 memo supporting public school segregation, his opposition to a public accommodations ordinance in Phoenix in 1964, his opposition to school desegregation proposals there in 1967, and his leadership of the "ballot security" programs in the 1960s that were "a euphemism for intimidation of black and Hispanic voters."

"Imagine what America would be like if Mr. Rehnquist had been chief justice and his cramped and narrow view of the Constitution had prevailed in the critical years since World War II," Kennedy said. "The schools of America would still be segregated. Millions of citizens would be denied the right to vote under scandalous malapportionment laws. Women would be condemned to status as second-class Americans. Courthouses would be closed to individual challengers against police brutality and executive abuse -- closed even to the press. Government would embrace religion, and the wall of separation between church and state would be in ruins.

" . . . That is not a vision of America I support, nor is it a vision the vast majority of our people would support," Kennedy said. "Justice Rehnquist is outside the mainstream of American constitutional law and American values, and he does not deserve to be chief justice of the United States."

Chairman Thurmond, by contrast, said he felt that in 14 years on the court Rehnquist had displayed all the requisite qualities: "unquestioned integrity . . . courage . . . a keen knowledge and understanding of the law . . . compassion . . . proper judicial temperament . . . and appreciation for the majesty of our system of government."

As chief justice, Rehnquist would become, as Thurmond noted, "first among equals," presiding over court conferences, assigning the writing of opinions when he is in the majority, and making hundreds of judicial assignments as the head of the federal court system. Thurmond said he was confident that Rehnquist had the leadership quality essential to handle the task.

Rehnquist spent only a couple of minutes before the Judiciary Committee, telling members that it was "a great honor" for him to be there, and adding that he was "deeply grateful to the president for the confidence he manifested in me when he nominated me."

Sen. Alan K. Simpson (R-Wyo.) told Rehnquist that the rest of the proceedings would be like riding "a Brahma bull." He told the nominee to expect "loose facts, nastiness, hype, hoorah, maybe a little bit of hysteria" and to "be prepared to hear himself described as a racist, extremist, trampler of the poor, assassin of the First Amendment . . . and a crazed young law clerk who was about two tacos short of a combination plate."

The only witness to give any substantive testimony was President Jimmy Carter's attorney general, Griffin B. Bell, now an Atlanta lawyer. He urged the committee to approve the nomination.

Bell said Rehnquist was a leader on the court "because of his powerful intellect . . . . I do not consider him an extremist," Bell said. "We don't have one on the court." He likened Rehnquist's conservatism to his polar opposite on the court, Justice Brennan. "Both have strongly held views," Bell said, but "render reasoned decisions."

Nearly 50 witnesses are waiting to testify after Rehnquist is done. Sen. Howard M. Metzenbaum (D-Ohio) said he also wants a dozen witnesses from Phoenix to help the committee resolve allegations that Rehnquist personally challenged minority voters at the polls there, particularly in the 1962 election, demanding that they prove their literacy by reading passages from the Constitution on cards the GOP had printed up.

Rehnquist said in 1971, when he was first accused of voter harassment, that he never personally engaged in challenging any voters in any election. The Federal Bureau of Investigation has just completed an inquiry into the renewed controversy and submitted a thick report to Thurmond. A Thurmond spokesman said he will make no decision on calling the Phoenix witnesses until he has reviewed the FBI's findings.

Hatch alluded to recent battles before the committee over President Reagan's judicial nominees, such as Daniel A. Manion and Jefferson B. Sessions, and said he thought it was time to call a halt to the warfare.

"I believe it's time we stop hacking at everyone who comes before the committee . . . and stop the character assassination that's been going on," Hatch said.

Critical Democrats took the position that they were interested only in the facts. Metzenbaum, for instance, said that a clear pattern emerged from Rehnquist's opinions on the high court.

"If the issue involves individual civil liberties," Metzenbaum said, "if the issue involves a criminal defendant's rights, the defendant's claim is likely to be denied. But if the issue is whether big government is going to get its way, the result is likely to be that it will . . . .

"There is no doubt that the president should have wide discretion to pick nominees," Metzenbaum said. "He won that right a year ago last November. But there was no electoral mandate to repeal basic constitutional values. There was no great cry throughout the land to cut back on the Bill of Rights.