A rainbow-hued burst of balloons rose skyward above the 315-foot clock tower of the Old Post Office Building yesterday at an outdoor ceremony to mark the beginning of the $18-million restoration of the Pennsylvania Avenue landmark.

It was an outdoor ceremony for an interior renovation because construction work already has begun and as General Services Administrator Jay Solomon noted, impatience had led to a tragedy 79 years ago.

"The first post mastergeneral was as eager as I to get in the building . . . and fell down an elevator shaft," Solomon recalled.

On Sept. 30, 1899, at midnigth, the new building was turned over to the Post Office Department and during that acceptance ceremony Washington Post master James P. Willett stumbled into an unguarded elevator opening and fell to his death. tr for add 1

So yesterday, the ground-breaking festivities were held outdoors with construction workers on the job inside.

First Lady Rosalynn Carter and Joan Mondale both were there to join in the celebration.The restoration, which will combine federal offices with public use for shops, restaurants and cultural activities, is designed to bring new life and energy to Pennslyvania Avenue.

On the speakers' stand at 12th Street and Pennsylvania Avenue NW, Mrs. Carter spoke of this "main street in the nation's capitol," adding: "and like main streets all over the country it needs restoring."

The Romanesque-style Post Office, the first building constructed on what was to become the Federal Triangle, has escaped demolition several times in recent years. Yesterday the clock in the tower was on time, as if it knew it had a future; for many years, the clock was notorious for not keeping correct time. The heavy hands gained at the beginning of each hour as they fell from the number 12 and then lost time on the upgrade.

The Old Post Office restoration, scheduled to be completed by late 1980, is the first project under the Public Buildings Cooperative Use Act of 1976.

"We cannot affort to build them like this any more, and in the past we have been too quick to tear them down." Mrs. Mondale said.