President Reagan yesterday described his administration as having the best record of any for aggressively pursuing violations of minorities' civil rights despite what he said is his public image as a president who does not support laws against discrimination.

In a live, half-hour lunchtime radio interview, the president defended his administration's intervention in a Supreme Court case on the side of white policemen and firefighters in Boston. They are challenging a lower court ruling that they should be laid off before blacks and Hispanics with less seniority to maintain the level of minorities in the police and fire departments.

Reagan said that allowing the lower court's ruling to stand would verge on government approval of quotas for minority groups, which he noted had been used against minorities in the past and could be used against them in the future.

"I'm old enough to remember when quota systems and so forth were used, not to end discrimination but actually to legitimize it," the president said. "And the precedent that would be set if this policy . . . is used in this instance admittedly to try to keep a better balance, which we're all in favor of in public employment in that area, but if that precedent is set, that seniority is meaningless . . . some day you could find an administration that wants to turn it around and use it to go back to what we hopefully have gotten rid of in this country, a discrimination against minorities."

The rare exclusive interview for radio follows a spate of published interviews and impromptu appearances before the press during the past two weeks, as the result of growing criticism of Reagan for being isolated and avoiding the press. In the radio interview, the president also:

* Repeated his threat to veto either the $5.4 billion jobs creation bill passed by the House, the $1.2 billion Senate bill or any compromise of the two in conference committee, and promised to work to try to get his proposed nickel-a-gallon gasoline tax bill passed.

* Said the Soviets have better military "firepower" than the United States but that he would not trade military forces with the Soviets because of his faith in "America and the young American men and women who are in the armed forces."

* Complained that the news media are overdramatizing the extent of unemployment in the country with the result of "psychologically" harming his administration's efforts to create an economic recovery.

* Said he thinks differences dividing the special commission studying the Social Security system "are not all that great," although they have yet to make a recommendation to him and some members have asked him to give them an idea of what proposals he would like to see in their final report. Reagan did not specify what differences are slowing the commission's work.

* Said that although he has commissioned a group to review alternatives for the MX basing system, he is not committing himself to accepting their recommendation. "Well, it would depend on whether there was other agreement with the commission . . . by that I mean the expertise of our own military people who are going to have to use these weapons if they ever are used."

* And said he instructed Philip C. Habib, the U.S. special envoy to the Middle East, to go beyond talks of a peace plan and look for a "plan for action." The president added that he wants American troops withdrawn from Lebanon but has no timetable for the action.

"The main thing right now we have Ambassadors Habib and Morris Draper working on in the Middle East is to get what now constitute armies of occupation -- the PLO, the Syrians, and the Israelis -- out of Lebanon and let the Gemayel government have the sovereignty of their own country . . . for them to continue to stay against the will of Lebanon makes them, technically, armies of occupation."

In the interview conducted in the White House's Roosevelt Room, named for President Franklin D. Roosevelt, Reagan -- himself a former radio announcer -- also said he assumes he will announce "sometime next year" whether he will seek a second term.

The president added: "One way you're going to become a lame duck and have no authority to do anything you're trying to do. Or the other way you're going to open yourself to opposition charges that everything you do is political . . . "

The interview was conducted by Ted Clark of National Public Radio, Bob Ellison of Sheridan Broadcasting Network, Joe Ewalt of RKO Radio Network, Gene Gibbons of United Press International and Frank Sesno of the Associated Press.

The White House could not back up Reagan's claims about the supremacy of his administration's record on pursuing cases of civil rights violations.

Reagan's remarks on his civil rights record came in response to a question about why he backed Justice Department intervention in the Boston case on the side of the white police and firefighters instead of adopting the optional proposal of having minorities and whites laid off in proportional numbers.

" . . . An image has been created of me, I know, that I do not support these anti-discrimination measures," Reagan said. "The record, on the other hand, proves the reverse . . . .

"Here we have already, and we haven't been here two years yet, we have 130 members of the black community in top executive positions beginning with Housing and Urban Development Secretary Sam Pierce, a very capable member of our Cabinet . . . and then added to that we're doing the same thing with regard to women. And we're doing the same thing with regard to Hispanics."

An aide supported Reagan's claims, citing statistics on government involvement in civil rights cases. Through mid-September the Reagan administration has filed 62 new criminal civil rights cases, according to press aide Anson Franklin, and 52 cases have been conducted. The administration has also been involved in 32 voting rights court cases.