Twenty years after his death, slain civil rights leader Medgar Evers was remembered yesterday at Arlington National Cemetery as "a man who left the world better than he found it."

That description of Evers came from NAACP executive director Benjamin L. Hooks, who said many of those attending yesterday's memorial service would never understand the tension that accompanied Evers' every moment as the NAACP's first field director in Mississippi.

The special service, the first at the gravesite since Evers was buried there, drew about 100 people, including his widow, Myrlie, other civil rights activists, Sen. Thad Cochran (R-Miss.) and assistant attorney general William Bradford Reynolds, who used his attendance at the event to try to blunt criticism of the Reagan administration's civil rights policies.

Reynolds, who has stirred controversy among civil rights leaders for his opposition to mandatory busing and affirmative action quotas, said Reagan's civil rights policies have been "terribly misunderstood" because there had been "a failure to communicate the good things."

Reynolds, who did not speak at the ceremony, is scheduled to visit Mississippi this week to investigate allegations of voting irregularities in the state.

Myrlie Evers, whose husband was only 28 when he was shot in the back while walking from his car to the door of his house in Jackson, Miss., helped to lay a single wreath at the foot of his grave, and a lone soldier played "Taps."

Hooks and several other guests at the service, including Cochran, eulogized Evers, an Army veteran who had just led a month-long antisegregation drive at the time of his shooting on June 12, 1963. Mayor Marion Barry did not attend the service but sent a proclamation declaring June 12 Medgar Evers Day in Washington.

After the service, Myrlie Evers said that the Reagan administration, "in its effort to dismantle the efforts we have made has served as a warning to black people." She said that her late husband had often spoken of such warnings that serve to awaken people and show them they "must guard the gains they have made on a daily basis or they will be taken away by other people or by the government."

Evers is the executive director of community affairs for Atlantic Richfield Company.

Hooks, answering questions before the service, said the NAACP Board of Directors' decision to displace Margaret Bush Wilson as chairman of the organization was "not an issue of Hooks versus Wilson but Wilson versus the Board." He called Reynolds' appearance at the service "an irony . . . but I would hope that something said here today will touch a note."