Estimates of the turnout in yesterday's election indicate that Cambridge Convention '75's strategy of increasing registration among young voters may pay off with a fifth liberal seat on the City Council.

Approximately 28,000 people voted in the election, Edward J. Samp Jr., a Cambridge election commissioner, said last night--about 2000 more than turned out for the 1973 municipal election.

He said the voting pattern appears to be favorable to Cambridge Convention '75.

Samp added, however, that a breakdown by district reveals that although many Harvard Students registered to vote few showed up at the polls yesterday.

By tomorrow evening the commission will have the results of an informal count of the ballots in the City Council race. "We should have a pretty good idea who's going to be elected then," Samp said.

Pretty High

The turnout was "pretty high," Cambridge Mayor Walter J. Sullivan, a member of the conservative Independent group, said last night, and he conceded that "would give some favor to Cambridge Convention '75."

Sullivan added that although it is too early to predict the outcome of the race, there is no doubt that the "big voter registration" will "make a difference" in the vote.

Cambridge Convention '75, a coalition of nine liberal candidates for the city council, enrolled nearly 5000 new voters before the election, mostly in the Central and Harvard Square areas.

By presenting a unified slate and increasing their constituency, the candidates hope to tip the political balance in their favor and capture five of the nine seats on the council.

Throughout the race, convention leaders have maintained that a high turnout in this year's basically issueless and lackluster campaign would indicate success for the Convention.

David Sullivan, one of the convention's organizers, said last night that things were "looking really good; I think we are going to win."

The only major voting irregularity alleged yesterday was a challenge by Cambridge Convention '75 of 150 absentee ballots filed in East Cambridge, a traditional Independent stronghold.

Francis H. Duehay '55 a convention-endorsed incumbent, said last night that the challenged votes were from residents not entitled to file absentee ballots because they were neither ill nor out of the city on election day.

"Some candidate apparently went around signing up people with absentee ballots," Duehay said.