ONE OF THE continuing mysteries of prosecution in the capital city is a dual standard of treatment by U.S. Attorney Joseph diGenova's office of people who choose to demonstrate in front of embassies here. It's been going on for more than a year now and has yet to be explained satisfactorily. It boils down to this mysterious difference: if you demonstrate in front of the Soviet Embassy, you're likely to be charged, prosecuted and punished, but if you demonstrate in front of the South African Embassy, you may be charged but the charges are dropped before you get near a court. What kind of interpretive latitude is this?

We're not talking isolated instances. In March 1985, we noted that some 1,665 people had been charged with breaking a law by demonstrating within 500 feet of the South African Embassy. Mr. diGenova had seen to it that charges were dropped before anyone went to court. The number has since passed the 2,000 mark, and the policy hasn't changed. Meanwhile, those who protest within 500 feet of the Soviet Embassy are going to court; just the other day, 21 students were convicted, given 15-day suspended sentences and ordered to pay $60 each in fines and court costs. What's the difference?

In a letter last summer to a House Judiciary subcommittee chairman, Mr. diGenova cited legal precedents that he said allow discretion in choosing how or whether to prosecute. He says this can take into account the wishes of ambassadors at the embassies where the incidents occur and the possibility of retaliation against U.S. embassies if protesters here are let go without charges.

At that rate, you could wind up with 100 different treatments of protesters, each tailored to the whims of a different ambassador around town. It might be a politically convenient arrangement, but it's no way to enforce a law that applies uniformly to every embassy -- and should be enforced with some semblance of equity.