Dwight S. Cropp, whose name is scarecely a household word, became in effect the acting mayor of the District Columbia yesterday for a term that will last just four days.

Cropp, whose regular job is executive secretary of the D.C. government, was the highest ranking official left in the city when Mayor Marion Barry led an exodus of 70 department heads and other key personnel for a management seminar at Berkeley Springs, W. Va.

Oridinarily when the mayor leaves town, his powers and duties - but not his title - are exercised by the city administrator. If the city administrator also is gone, the powers go to the corporation counsel.

Since City Administrator Elijah B. Rogers and Corporation Counsel Judith W. Rogers (who are not related) both went to the seminar, Barry signed an order giving the stopgap powers to the city's executive secretary.

The mayoral powers are rarely used in such instances, except in emergencies.

Tongue in cheek, the 39-year-old Cropp said he planned no special initiatives in the job, and would neither veto any legislation passed by the City Council nor occupy the mayor's large corner office.

"I'm just going to report to work and pray that nothing unusual happens," Cropp told a reporter. Asked how he felt, he replied, "Very strange - I never would have believed it, growing up as a kid in this town."

Cropp, a lifelong Washingtonian who was graduated from Spingarn High School and Howard University, was executive secretary to the Board of Education for five years before Barry appointed him to his present job in January. The post involves an array of administrative and ceremonial tasks.

Before leaving, Barry held a sidewalk news conference in which he said the seminar - which will cost about $10,000 - should pay off in improved services and teamwork among city officials.

Barry said he expects to sign on Monday the emergency bill passed this week by the council removing the 11 percent ceiling on mortgage loan interest.

The 70 officials traveled in two school bus-type vehicles and a van owned by the D.C. Department of Human Resources. The van's battery went dead while waiting outside the District Building, and had to be started by a jumber cable attached to a television cameraman's car. CAPTION: Picture, Dwight S. Cropp is acting mayor of the city, a term that will last four days. By John McDonnell - The Washington Post