Howard University law professor and attorney Alex Williams formally announced yesterday that he will challenge five-term incumbent Arthur A. Marshall Jr. for Prince George's County state's attorney in the September Democratic primary election. Marshall faced his last primary opponent in 1970.

Williams, 37, vice chairman of the Washington Suburban Sanitary Commission, told more than 400 supporters yesterday at Prince George's Community College that his candidacy will "unite the community." He is the first black Democratic candidate for countywide office in Prince George's, according to a local planning agency historian.

"The very fact that I stand before you today as a credible candidate for a major office is a clear indication that this county has changed," Williams told the largely black crowd of fellow church members, Democrats and local officials.

Also on hand was Rep. Michael D. Barnes (D-Md.), who is running for U.S. Senate. "This is a very important beginning for something that is obviously going to have a significant impact in Prince George's County," said Barnes, the only statewide candidate in attendance.

Thomas Farrington, an attorney who for years was associated with the county's dominant political machine, told the audience that it is time for county voters to elect a black to countywide office.

"It's an insult to everyone in Prince George's County to think that the state of Virginia is ahead of Prince George's County," he said, referring to the recent election of Virginia Lt. Gov. L. Douglas Wilder, who is black. "The time is now, because people can read the numbers and they're not stupid."

Based on U.S. Census projections, blacks will constitute a majority in Prince George's by this summer, a tipping of the scales that could benefit an attractive black candidate.

Alvin Thornton, a county activist and Howard University political scientist, stood on the fringe of the gathering and speculated that any strength that Williams has in Prince George's could prove attractive to those with the power to assemble political slates.

"Having Alex Williams on a slate is an instant inroad into the belly of the black community," said Thornton, who noted that even black candidates who are challenging each other for local office are supporting Williams.

Roy Dabney, one of three leaders of state Attorney General Stephen H. Sachs' gubernatorial campaign organization in the county, said that Sachs is hoping to work out a political "marriage" of convenience during the campaign. "On election day, everybody's going to need everybody," Dabney observed.

Absent from yesterday's festivities was County Executive Parris Glendening, who appointed Williams to the sanitary commission position. In addition, there were no representatives of Baltimore Mayor William Donald Schaefer, another likely candidate for governor.

It was in Baltimore in 1982 that black candidate Kurt Schmoke, in a race Williams' supporters cited as an example to be copied, beat a white incumbent state's attorney in a landslide that many observers attributed to the unified strength of the city's black vote.