Without a single person entering a polling booth, the City of Cambridge elected a new city councilor this week and set the stage for a mayoral election later this month.

Alfred W. LaRosa was elected Wednesday by Cambridge's unusual system of proportional representation to fill the City Council seat of the late Mayor Leonard J. Russell, who died on June 16.

LaRosa gained his seat after the 2899 ballots Russell won in the last election were reopened, and the votes recounted to determine the next-best finisher from the seven candidates who didn't win council seats in that Nov. 1983 election.

The full nine-member council will meet to select a mayor from within its ranks, "as soon as we can get people to agree on a date," Vice-Mayor Francis H. Duehay '55 said yesterday.

All the councilors, except Saundra M. Graham, who will be out of town, will attend LaRosa's swearing in on June 15, Duehay said.

LaRosa was pronounced the winner after 10 election commissioners and staff members spent 13 hours sorting the ballots. LaRosa received 502 votes, while Richard P. Branson finished second with 327, and David Wiley, third with 322.

Mixed Emotions

"I am elated to get the position, but saddened because Lenny Russell was a friend," said LaRosa, a resident of East Cambridge.

LaRosa, who ran as an independent and has a neighborhood-oriented, conservative constituency said he hopes to "play Russell's role as a good listener."

LaRosa added that he is particularly concerned about the encroachment of housing, industry, and retail establishments into Cambridge neighborhoods.

"I want to make sure that Cambridge is for the people who live there," said the 42 year-old LaRosa, who has worked as a researcher for the State Legislature for 14 years.

"We are fortunate to get a person of such broad experience," Duehay said of LaRosa, who has also served as vice-president of the East Cambridge Planning Team.

The recount was held in a gymnasium in the Cambridge Police Station in front of a small crowd of candidates and other onlookers. The commissioners and other staff members spent all day and into the night sorting through the ballots, to eliminate the ones that held votes only for Russell and other victorious candidates.

"This is Cambridge political hackery at its best," said spectator Eric Wolf, son of Councilor Alice K. Wolf.

Political Jockeying