There were jesters on the front yard, and as the guests crossed a bridge, a group of Renaissance singers standing in the darkness began to sing. It was just one instance of the mystery and fantasy woven for last night's Opera Ball at the Royal Swedish Embassy.

The benefit was billed as "a night at Gripsholm Castle," and it seemed that the crowds of long-gowned women and black-tied men would p.m., and one guest who showed up on time said that at first the place was empty. But soon, she said, it was "like they're getting off a Greyhound bus."

There was actress Liz Taylor in a deep blue gown and her husband John Warner. Rep. John Brademas (D-Ind.) exited the receiving line without a lady on him arm. A long-time bachelor, he recently became engaged. In the next room, Sen. Edward Brooke (R-Mass.) was chatting with a group of friends.

On the other side of the room, Taylor's progress was halted at the sight of a green parrot. The parrot, whose name is Fifi, belongs to florist Henner, Wachs, the owner of Der Blumenstrauss and the man who had done all the floral arrangements for the party. Taylor told Wachs about how she'd lost her parrot in California when it was put with the baggage and Henner promised her he'd get her one.

While the crowd at the door, a sea of silks and billowing dresses, waited to meet the ambassador, Count Wilhelm Wachtmeister, and Ina Ginburg, chairman of the ball, Jane Ikard, a member of the benefit committee, scanned the room for Regine, a famous discoteque owne.

"Does anybody know what she looks like?" she queried. "I don't know what she looks like."

Elsewhere in the party, Robert J. Lipshutz, consel to the President, and Phillip Buchen, counsel to former President Ford, were sitting at a table discussing their job. Mrs. Ikard said they had wanted to meet for a long time, so she introduced them. "Now they're talking shop," she said.

Other guests had put official Washington aside for the evening. Peter Duchin's band had guests tapping their toes and snapping their fingers.

In and out of the crowd waiters and waitresses scurried, the women dressed in Scandinavian countryside costumes.