BOSTON--Less than a third of Boston's 207,000 registered voters are expected to turn out in today's non-partisan mayoral primary which is likely to produce a general election showdown between Mayor Raymond L. Flynn and City Councilor Joseph Tierney in November.

City elections officials yesterday predicted a turnout of 15 to 30 percent of Boston's voters in the so-called "preliminary" election in which Flynn and Tierney are vying with two lesser-known candidates to become the top two vote-getters to advance to the Nov. 3 general election.

The two other candidates on the ballot, who have been running low-profile campaigns, are Joel San Juan and Richard A. Black, a follower of controversial presidential hopeful Lyndon LaRouche.

Flynn, 48, is seeking his second term as mayor of the nation's 20th-largest city and hub of the country's 10th-largest metropolitan area. Tierney, 46, is making his first run for the $100,000-a-year job after 16 years on the City Council, 10 years as its president.

Polls have shown Flynn to be at the peak of his popularity in the city and political observers have said his election come November is almost a sure bet. A former schoolteacher, Flynn is known for his down-to-earth, neighborly style. He has been praised for reaching out to all the neighborhoods in the city.

Flynn spent yesterday welcoming former Israeli Prime Minister Shimon Peres to the city and attending a groundbreaking with Gov. Michael S. Dukakis for a $20.3 million, 165-unit housing project in Boston's Mission Hill section.

The project, the largest state-funded project of its kind in the city, is for first-time homebuyers and low-income renters. Flynn called it "truly a national model."

Tierney, considered more conservative than Flynn, spent the day "out in the community" campaigning, said campaign director Evelyn Friedman, who described Tierney's chances heading into the preliminary as "great."

Flynn has raised $1 million for the race, while Tierney has collected $118,000.

Tomorrow also will be the preliminary council election, which will narrow down the field for the November election for the nine spots on the Boston council. Political observers say that these races are key to Flynn, who is looking to gain his first majority on the council. The issue of rent control, which is supported by Flynn, has dominated the debate during this campaign.

Boston is among 15 Massachusetts cities holding preliminary elections for local officials Tuesday. The others are Beverly, Everett, Haverhill, Lynn, Leominster, Medford, Newburyport, North Adams, Peabody, Pittsfield, Revere, Salem, Springfield and Westfield.