It was hard to tell who was lying in wait for whom in the Blue Room at the White House last night. With West German President Karl Carstens at his side craning to hear the exchange, President Reagan tried to set a few things straight with reporters about his aborted trip to the Philippines.

"I was really mad," he said, when he read news reports that he had canceled part of his trip, and he'd been waiting all day to say so. In the reports, administration officials who asked not to be identified cited the instability of the Marcos regime. There were also reports that Nancy Reagan was worried about the safety of the president, who was wounded in an assassination attempt two months after he took office. Yesterday, in Manila, President Ferdinand Marcos said he understood Mrs. Reagan's concerns.

"I was most disappointed with the press this morning," the president said, casting a stern look at three reporters who had gathered around him while his other 125 guests sipped coffee and liqueurs after the state dinner for Carstens and his wife, Veronica.

"We made a very honest decision and an honest statement. In fact, we didn't cancel anything. We postponed it because Congress would be in session," he said, "and the first time I learned they were, I said, 'I don't see how I can possibly be gone.' "

Reagan added, "Our security has never said to me that they can't guarantee security." And he expressed concern that although he had dispatched a special messenger to convey his regrets to leaders in the Philippines, Thailand and Indonesia, "they'll read accounts here, I'm sure they will, and it could be very embarrassing."

He said the trip, orginally scheduled for Nov. 2 to Nov. 16, was "very long." Now, only Japan and Korea will be visited and the other three nations rescheduled. He also plans to go to the People's Republic of China next April.

The discussion of security prompted actress Ginger Rogers to tell the group and the president, "Oh, I get scared for him" and, turning to Reagan, said, "because I love you so much. And we who love you get scared for you.

"I think the press is terrible," she said. "They've not been very nice to our president."

Reagan didn't show his distress earlier when he and Mrs. Reagan welcomed the Carstenses at the North Portico. In fact, while waiting for them, the Reagans practiced up on their dancing. Without musical accompaniment and oblivious to the stares of reporters outside windows, they twirled around the grand foyer for three or four minutes.

After cocktails upstairs in the family quarters, Reagan and Carstens, followed by their wives, began the ceremonial descent down the grand staircase. From the vantage point of Franklin Roosevelt and Woodrow Wilson, whose portraits hang there, there might have been some ironies in the occasion. Both presidents were commanders in chief of American troops in the two wars against Germany, and when Carstens noticed the Wilson portrait, he motioned towards it.

The atmosphere was sehr gemuetlich, or very friendly. And that feeling carried over into the toasts, taken note of by both Reagan and Carstens on the occasion celebrating the arrival 300 years ago of the first German settlers in America.

In his remarks, Carstens said, "If the alliance endeavors to obtain a military equilibrium at as low a level as possible, this will guarantee not only freedom but also peace. Both freedom and peace would be endangered if the other side were to acquire military superiority."

He acknowledged what he called the "charitable assistance" of the Americans after World War II, including the economic aid provided by the Marshall Plan and the 1948 airlift to Berlin, "a city which owes so much to the American people."

In some respects, it seemed to be an evening of dancers. There was Martha Graham, the doyen of modern dance. And on the dance floor after the entertainment, there was Ginger Rogers again, first with Vice President Bush and later with Secretary of State George Shultz.

Rogers said she appeared in one movie, "Storm Warning," with Reagan but that they did not dance. "Hardly," she said, pointing out that of her 73 movies, only 15 were musicals.

Another on the dance floor was Vanessa Williams, the 1983 Miss America, whose escort for the evening was Air Force Maj. Ivan Kelly.

Kelly's role, under a new White House escort policy for single women, required him to call for Williams at her hotel in a White House car. Social secretary Gahl Hodges said the practice was started during the dinner for the Portuguese president because Mrs. Reagan "felt it was the gracious thing."

"It's like going to a prom," said Williams, wearing a spectacular sequined white gown, with one shoulder bare.

"I don't know if it was accidental or not but it was a perfect match," said Hodges of Kelly, one of 25 White House social aides on duty last night. Another White House source said, "I think he volunteered. Wouldn't you?"

At dinner where she was seated at the president's table, Williams said Reagan told her what Hollywood had been like "in its golden age."

In addition to Rogers, two other actresses in the crowd were Helen Hayes and Jaclyn Smith. Hayes said she didn't know how many times she had been to a White House dinner but Smith said she did. Once.

Rep. Olympia Snowe (R-Maine), another first-timer, said she thought it was "just an invitation" that had nothing to do with the new White House campaign to win the women's vote. Snowe, who is head of the Republican Women's Caucus, said her group had been meeting regularly with the White House on such issues as dependent care, spousal IRAs and aspects of the Women's Economic Equity Act.

"But we still have a ways to go. We were scheduled with meet with Michael Deaver this week," she said of the deputy chief of staff, until he flew to the Far East last Friday on his presidential mission.

The four-course dinner of filet of sole in aspic, roast tenderloin of stuffed veal, endive and watercress salad with assorted cheeses, and hazelnut autumn log was served on the Reagan china. After dinner, entertainment in the East Room was by Sherrill Milnes, considered one of the world's leading operatic baritones.

Mrs. Reagan wore a navy and red sequined gown with tunic top. At her throat was a imitation garnet choker by Kenneth Jay Lane, one of several designers in the crowd. (Halston came with Martha Graham.) Speculation mounted on who had designed the first lady's gown, but aides weren't certain. Designer Pearl Nipon, equally glamourous in one of her own designs--a black gown with tucks--laughingly offered a suggestion.

"Why don't you just say it's a Nipon, and I'll deny it?"