Henry E. Hudson was sworn in yesterday as U.S. attorney for the Eastern District of Virginia, as Helen Fahey formally announced her candidacy for the Arlington County prosecutor's position Hudson vacated.

Hudson, 38, was described as "an outstanding law enforcement officer" by Sen. Paul S. Trible Jr. (R-Va.), who nominated him for a four-year term in the $70,500-a-year post.

About 150 well-wishers crowded into the Alexandria federal courtroom where District Judge Claude M. Hilton administered the oath of office to Hudson, a Republican who headed the U.S. Attorney General's Commission on Pornography.

Chief Judge of the Eastern District Albert V. Bryan Jr., who presided at the brief ceremony, welcomed Hudson "into the court family."

"We may not have a full menu of pornography cases here for you, and we are not looking for a full menu of such cases," he said.

Fahey, a Democrat who was Hudson's chief deputy, succeeded him as commonwealth's attorney on May 23 after being appointed by Arlington's four Circuit Court judges to serve until the November election, when voters will select a prosecutor to fill the one year left on Hudson's four-year term.

She faces a vigorous contest for the Democratic nomination against Brendan Feeley, a former assistant prosecutor. Arlington Republican leaders say they do not intend to field a candidate.

In recent debates before the Arlington Young Democrats and the Arlington Bar Association, Fahey's and Feeley's experience emerged as the primary issue in their contest for the $55,000-a-year county job.

Fahey said yesterday she would stress recruitment of minority attorneys and prosecution of white-collar fraud against the elderly.