NEW YORK (AP) _ Mayor Edward Koch's newest Democratic challenger may not be instantly recognizable, but Herman D. Farrell Jr. says he wasn't elected Manhattan county leader three times without ''the support of labor, Latinos, blacks, women, progressives, of gays, of lesbians.''

''I was able to do that because I was able to work with people,'' Farrell, an assemblyman, said Monday on the steps of City Hall when he officially announced he would oppose Koch in a Democratic primary.

Farrell was endorsed Friday by the Coalition for a Just New York, a group of 34 black leaders and clergymen.

Meanwhile, City Council President Carol Bellamy was scheduled to make a tour of the five boroughs today to announce her intention to run against Koch.

Farrell, a black and a state assemblyman for 10 years, said he had sent a telegram to Koch asking for a meeting to discuss ''how we can both stay on the high road'' during the campaign ''for the eventual unity of the city.''

Koch said at a news conference later, ''All he has to do is stay on the high road. He'll find me there.''

Farrell told reporters Koch has not delivered or improved city services as he had set out to do when first elected in 1977.

''What do you expect him to say?'' Koch shot back when asked about it by a reporter. ''Do you expect him to say nice things?''