Lawyer John L. Ray began the final phase of his campaign for an at-large D.C. City Council seat in the May 1 election yesterday with a $10,000 radio advertising campaign that features an endorsement from Mayor Marion Barry.

Three separate 60-second radio ads began airing yesterday on nine area radio stations with a theme of "John Ray Works." That theme is tied into Ray's campaign strategy to paint former council member Douglas E. Moore, the other major candidate for the at-large council seat, as someone who may share Ray's position on some key city issues, but will be unable to accomplish anything on the council.

"The key issue here is who offers the voters the best chance for getting something done about the problems they already know exist," Ray said, explaining his campaign strategy.

"You've got to have someone who can work with other memebers of the City Council and the mayor.And we need someone who works, period. Doug is basically lazy. He's a great position taker," Ray said.

Moore refused yesterday to comment on the radio ads. He is expected to begin broadcasting his own radio ads later this month.

Yesterday, the Third World Alliance, a 300-member organization which recently has been identified with national opposition to President Carter's policies affecting blacks, announced its endorsement of Moore and William Revely, a candidate for city council from Ward 4.

Laced throughout the Ray ads is the phrase, "John Ray Works." The three ads also include endorsement of Ray's candidacy by neighborhood, religious and business leaders. Council member John A. Wilson (D-Ward 2), for example, calls Ray, "One of the brightest men that I know."

C&P Telephone Co. executive Delano E. Lewis, who directed Barry's transition team, refers to Ray as a "voice of reason."

The Ray ads were prepared by Abramson-Himmelfarb Inc., the same firm that developed the advertising package that accompanied Barry's campaign for mayor.

The other candidates in the at-large council race are Jackson R. Champion, Stuart Rosenblatt, David G. Harris, Lin Covington, Richard Blanks Sr., Frances Goldman, Warren A. Hemphill Sr., Hector Rodriquez and H. Chris Brown.