Riding in a limousine led by two police motorcycles with red lights flashing and sirens wailing, D.C. Mayor Marion Barry took the chief executive of one of West Africa's largest cities on an unusual guided tour of Washington yesterday.

The itinerary for Lamine Diack, mayor of Dakar, the capital of Senegal, included some of Washington's least likely tourist attractions.

It began at the sprawling Blue Plains sewage treatment plant in far Southwest Washington and ended three hours later with a visual sweep of 14th Street NW, where Diack commented with favor on the new housing that is rising and diplomatically made no mention of hookers and junkies congregating on the corners.

Along the way, Diack saw a Metro train speeding across a viaduct above the Anacostia River alongside Benning Road and visited two housing projects -- the Fort Lincoln new town and the old Iowa apartment house on 13th Street NW, now being refurbished as a condominium. The two mayors dropped in for an unarranged visit with a startled housewife, Carolyn Duke, in a new 13th Street townhouse.

The French-speaking Diack listened intently as translator Hedda Brock relayed the descriptions offered by officials, workmen and residents who served as guides. He offered few comments.

Yesterday's tour was chiefly a return of hospitality the mayor received during his recent 19-day tour of Africa.

Diack guided Barry around Dakar. Barry invited Diack to return the visit. Diack decided to do so while on his way home from a recent international meeting in Montreal.

Yesterday's distinctly untouristy itinerary was arranged with a purpose, according to Guy Draper, Barry's newly appointed acting protocol director. Draper said the African mayor was interested in getting information that would be useful in guiding Dakar's development.

Barry said Dakar, a city of 1.1 million people, now dumps raw sewage into the Atlantic Ocean, and currently has an American firm consulting on possible plans for a treatment plant probably far less elaborate than Blue Plains.

Leading the mayor's three-car motorcade were two motorcycle-riding D.C. policemen, R. H. Kisner and Arthur Hill, who regularly escort the president and foreign dignitaries around the city.

As cars pulled to a halt, the mayor's blue limousine threaded between them at speeds up to 40 miles an hour, an American flag fluttering from its right fender and a red-and-white D.C. flag from its left.

Aulin E. Johnson, a veteran city chauffeur who drove for former mayor Walter E. Washington, was at the wheel. He said it was the first such extended excursion of its type since Barry took office in January.

At the motorcade pulled into Fort Lincoln, real estate saleslady Gwen Geddie gave the visiting mayor an animated greeting. She told Diack she recently returned from a vacation in a suburb of Dakar.