WASHINGTON (AP) _ A former federal prosecutor said today he saw William H. Rehnquist take part in a Republican campaign that ''was designed to reduce the number of black and Hispanic voters by confrontation and intimidation'' in Phoenix, Ariz., in 1962.

The testimony by James J. Brosnahan came after Democrats on the Senate Judiciary Committee, conducting confirmation hearings on the man nominated to be the nation's 16th chief justice, were denied access by President Reagan to memos Rehnquist wrote as an assistant attorney general more than 15 years ago.

The committee chairman, Sen. Strom Thurmond, R-S.C., gaveled down a request by Sen. Edward Kennedy, D-Mass., to consider issuing a subpoena for the memos.

Brosnahan, who was assistant U.S. attorney in Phoenix in the early 1960s, specifically contradicted Rehnquist's Senate testimony in 1971 that he never challenged the qualifications of any voters in Phoenix and was merely trying to arbitrate disputes at polling places.

''This does not comport with my recollection of the events I witnessed in 1962 when Mr. Rehnquist did serve as a challenger,'' Brosnahan, a lawyer from San Francisco, told the committee in a prepared statement.

Brosnahan, summoned to testify today by Senate foes of Rehnquist's nomination, said he saw Rehnquist at a polling place in a predominantly black and Hispanic precinct in south Phoenix during the November 1962 elections.

''Because the challenges were so numerous, the line of voters in several precincts grew long, and some black and Hispanic voters were discouraged from joining or staying in the voters' line,'' he said.

''It was my opinion in 1962 that the challenging effort was designed to reduce the number of black and Hispanic voters by confrontation and intimidation,'' he said, and that Rehnquist was actively involved in the campaign.

In testimony at his confirmation hearings in 1971 to be associate justice of the Supreme Court, Rehnquist described his role in a GOP ''ballot security'' project as arbitrating disputes at polling places. ''That is not what Mr. Rehnquist was doing when I saw him on election day in 1962,'' Brosnahan said.

The former prosecutor said he was summoned to the south Phoenix polling place to investigate complaints of illegal interference with balloting.

At the south Phoenix polling place, he said, ''the complaints did involve Mr. Rehnquist's conduct.''

The memos sought by committee Democrats were written by Rehnquist while he was an assistant attorney general and legal adviser to then-Attorney General John Mitchell from 1969 to 1971.

Democrats on the committee are seeking the memos because they purportedly concern domestic wiretapping and the Nixon administration's plans for dealing with Vietnam War protestors.

Thurmond, overruling the request for subpoenas, said: ''So far as I'm concerned, that's closed. This request is not reasonable, and I see no reason to pursue this matter further.''

He ignored a protest by Sen. Joseph Biden, D-Del., that 10 committee Democrats wanted to issue a subpoena.

At the White House, presidential spokesman Larry Speakes declined comment on Reagan's assertion of executive privilege.

The hearings resumed with representatives from civil rights and women's groups voicing their opposition to Rehnquist.

Eleanor Cutri Smeal, president of the National Organization for Women, said her group finds Rehnquist's views ''more than reactionary, we find them frightening.'' She added, ''This is not the record of a person who is supporting women's rights.''

Althea Simmons of the National Association for the Advancement of Colored People said Rehnquist's nomination has sent up ''red flags in Black America,'' asserting that ''he is opposed to civil rights.''

Kennedy earlier this week raised allegations that Rehnquist harassed and intimidated black and Hispanic voters in Phoenix.

In response, Rehnquist said he did not remember whether he ever challenged anyone's right to vote, a routine and proper endeavor by poll watchers, but denied he ever tried to block lawful voters from casting ballots.

Brosnahan was one of 10 witnesses the committee planned to hear from today in what Democrats said would be testimony that they saw Rehnquist try to intimidate minority voters.

Also summoned to testify were representatives of women's rights and civil rights organizations opposed to Reagan's choice of Rehnquist to succeed retiring Chief Justice Warren Burger.

Kennedy also disclosed that a deed for a suburban Phoenix home Rehnquist had owned from 1962 until 1969 contained a covenant forbidding its ownership, rental or occupancy by ''any person not of the white or Caucasian race.''

Thurmond, the committee chairman, served notice that the hearings would end early this afternoon and predicted that Rehnquist, an associate justice since January 1972, would be confirmed by the Senate.

''I don't know of anyone, anywhere, better qualified to be chief justice,'' Thurmond said.

But Kennedy accused the administration of ''stonewalling'' on the memos and said he would attempt to have the panel vote to subpoena the Justice Department files. Thurmond's response: ''I consider the matter closed.''

After Rehnquist ended his testimony last night, Assistant Attorney General John R. Bolton told the committee that the president, after consultations with Attorney General Edwin Meese III, had invoked executive privilege to block release of the Rehnquist memos.

Sen. Orrin Hatch, R-Utah, applauded Reagan's move and accused the Democrats of engaging in a ''fishing expedition'' for the documents in an attempt to discredit Rehnquist.

But Democrats complained that the decision would thwart a complete review of Rehnquist's qualifications.

Kennedy said the administration, by not releasing the documents, was ''doing a disservice'' to Rehnquist.

Sen. Paul Simon, D-Ill., referred to a 1984 Reagan executive order saying his administration would comply with requests for information and would claim executive privilege ''only in the most compelling circumstances. '' He said no such circumstances had been cited in denying the Rehnquist papers.

The latest covenant barring non-white ownership, uncovered by an FBI background investigation of Rehnquist for the committee, was written in 1929 for a home that Rehnquist owned in the Phoenix subdivision of Palmcroft. He sold the home in 1969, when he moved to Washington.

Questioned by Kennedy, Rehnquist said he couldn't recall whether he had ever read the deed. ''While very offensive, it (the covenant) has no legal effect,'' he said.

Democrats announced Wednesday that a vacation home Rehnquist bought in 1974 in Greensboro, Vt., contained a clause barring Jews from owning or renting the property.

Rehnquist said he was unaware of the restrictions in either deed until several days ago. He promised to take steps to have the anti-Semitic language removed from the Vermont deed.

Thurmond announced Thursday that an independent physician would review Rehnquist's medical records, interview his doctors and report the confidential findings to the committee. Rehnquist said he is healthy today after hospital treatment several years ago for a back ailment and the adverse effects of medication he once took for back pains.

Throughout the hearings, Democrats have questioned Rehnquist's commitment to women's rights and racial equality, and sought to portray him as a conservative extremist unfit to lead the Supreme Court after Burger retires.

Republicans are confident the committee will approve Rehnquist when it votes Aug. 14 on his nomination and that of federal appeals court judge Antonin Scalia to be associate justice.

Assuming the committee approves the Rehnquist nomination, it will go to the full Senate.