A D.C. Superior Court jury yesterday declared itself hopelessly deadlocked in trying to decide whether five community activists were guilty of destruction of property last July 1, when three of the group jumped into a mammoth apple pie unveiled by a group of Republicans on the Mall.

Judge Warren R. King declared a mistrial in the case and gave the government one month to decide whether it wanted to retry the group, all members of the Community for Creative Non-Violence.

In a separate charge stemming from the incident and heard by the judge alone, King yesterday found the group guilty of disorderly conduct and placed each on probation for six months.

Senate Republicans brought the two-ton pie to the Mall to commemmorate the first day of the Reagan administration's tax cuts. The pie, 17 feet in diameter, was billed as the world's largest and was to symbolize increased prosperity expected from Reagan administration policy. About 1,000 persons were on hand to listen to speeches and have a piece of the pie, donated by Safeway for the event.

The activists, who have become known at the courthouse as the "Apple Pie Five," arrived at the event dressed in business suits with pillows under their shirts. They purported to be "Republican Fat Cats", and wore signs identifying themselves as well-known, moneyed Republican supporters..

While two demonstators stood by, three others, one after the other, jumped into the pie just as Sen. William Roth (R-Del.), half of the team sponsoring the Kemp-Roth tax cut, was speaking. A Republican aide ran into the pie and tackled one of the demonstrators. The others were also dragged out of the pie by officials and arrested. The event went on, and people helped themselves to the pie, which had no upper crust.

All five demonstrators were arrested and charged with malicious destruction of property, which carries a penalty of one year in prison and a $1,000 fine.

Grace Lopes, a lawyer for the group, told the jury last week that the five did not intend to destroy the pie, only to "make a statement" by "altering a symbol they felt was a lie." Lopes argued that only a third of the pie was destroyed, and that "everyone there who wanted a piece of that pie got one."

To make her point, Lopes showed jurors a videotape of the event, recorded by the Repubicans, that showed children at the celebration happily eating pieces of the pie after the demonstration. Lopes said there were also some 700 pounds of leftover pie that were given to workers at a soft drink plant and to others for use as horse food.

But assistant U.S. Attorney Carolyn J. Adams, in her closing arguments, said the case also was about justice for the Senate Republicans who sponsored the event. Adams said the defendants had a right to protest, but not to destroy property. The event's sponsors had a right, she argued, to be free from destruction of their property.

The jurors deliberated nearly two hours Friday before recessing for the weekend. Yesterday morning they told King they were deadlocked. He urged them to continue deliberations. But late yesterday afternoon the jury reported it was unlikely to reach a verdict even if it continued deliberations.

Officials at the U.S. attorneys office said yesterday they were not sure whether they would ask for a new trial.