President Reagan today telephoned Martin Luther King Jr.'s widow to tell Coretta Scott King that he meant no offense when he said the American public won't know if the slain civil rights leader was a communist sympathizer or had communist associations until sealed FBI files are opened.

According to White House aides, Reagan made the call on his own at about 1:30 p.m. from the Oval Office to explain that he "did not want his remarks misinterpreted." An aide said King responded that "she understood." Reagan made the call before traveling here.

King said that Reagan "apologized to me for his remark . . . . He said it was a flippant remark that was made in response to what he considered a flippant question." She added that she "told him that I understand. I told him we all make mistakes and that I attributed it to human error."

After landing here, Reagan confirmed that he made the call. The president plans to spend the weekend playing golf with Secretary of State George P. Shultz, Treasury Secretary Donald T. Regan and former senator Nicholas F. Brady (R-N.J.). They will play at Augusta National Golf Course, which has no black members.

Asked if King accepted his apology in their conversation, the president nodded to reporters and said, "Yes."

However, White House spokesman Larry Speakes said Reagan did not apologize but simply offered an "explanation" that his remarks have been misinterpreted. A spokesman for King said the president told her "he was wrong."

The president did not say whether he would sign the bill that Congress has passed to establish Martin Luther King Jr.'s birthday as a national holiday at a public ceremony, which would give Coretta Scott King a forum for criticizing Reagan. Asked if she assumed she would be invited, King laughed and said: "They have no choice. I come with the package."

Earlier in the day the controversy about the president's stand on the King holiday deepened when former Republican governor Meldrim Thomson of New Hampshire disclosed the contents of a letter from Reagan in which the president said King's popularity is based on the myth that has built up around the civil rights leader since he was assassinated in 1968.

Reagan's letter was sent in response to a Sept. 30 letter from Thomson that asked the president not to sign the King holiday legislation. In the letter Thomson called King a man of "immoral character whose frequent associations with leading agents of communism is well established."

On Oct. 3, about two weeks before the Senate approved the King bill by 78 to 22, Reagan wrote that he shared the conservative Thomson's doubts about King.

"I have the same reservations you have," Thomson quoted the president as saying. "But here in Washington the perception of too many people is based on an image, not reality. Indeed the perception is reality. We hope some modifications might still be made in Congress."

White House aides acknowledged that Reagan wrote the letter but would not confirm the contents as disclosed by Thomson.

On Wednesday, in a nationally televised news conference, the president was asked if he agreed with the contention of Sen. Jesse Helms (R-N.C.) that King was a communist sympathizer or had communist associations.

Reagan replied: "Well, we'll know in about 35 years, won't we?" He added that he did not doubt Helms' "sincerity" in seeking to delay the Senate vote until the FBI files could be opened for congressional review.

The files were closed in 1977 for 50 years as part of a court settlement of a harassment suit against the FBI that was filed by the King family.

Reagan's remarks led King's widow to say yesterday that the president had insulted black Americans and that conservative opposition to the King holiday is "certainly strongly racial."

Administration officials today said that Reagan told top aides this morning that he understands the files contain no evidence of communist activities by King but that the transcripts of wiretaps and bugs placed on King by the FBI reveal King's marital infidelities.

"It was only a throwaway comment to get rid of the subject," a White House adviser said in describing the Reagan's explanation today of why he made the comment. "It was his way of getting away from the topic and reiterating that he supports keeping the records sealed . . . . If we felt any evidence was there of King being a communist you better believe we'd seek to open the files up."

The adviser added that Reagan's opposition to the King holiday has never been based on suspicions that King was a communist or a sympathizer but was founded solely on the cost of the holiday to the economy.