Boston — Geraldine Ferraro spoke not a word about abortion during Wednesday's rally outside Boston's City Hall. But judging from the posters in the audience, the hotly debated issue is what brought many of the estimated 10,000 spectators to the rain-swept plaza where the vice-presidential candidate was speaking.
Opponents of abortion have appeared at Ferraro rallies across the country, sometimes heckling the candidate during her speeches. There were plenty of posters at the Boston rally, too, taking the New York congresswoman to task for supporting a woman's right to choose an abortion.
But there also were just as many signs waved by those who supported Ferraro and the pro-choice position. Debbie Marranca of Boston, carrying a ''Catholics for Ferraro'' placard, said many Roman Catholics feel the same way Ferraro does about abortion. She and colleague Wendy Berg are members of Mass Choice, an affiliate of the National Abortion Rights Action League. They said their group, after hearing reports about hecklers in other cities, made an ''organized effort'' to counteract them at the rally here.
With hecklers silenced, usually shouted down by Mondale-Ferraro supporters, the Democratic vice-presidential candidate was able to turn her attention to captivating the audience. In this, she had some help from the Kennedy family.
First, US Sen. Edward M. Kennedy (D) of Massachusetts, who introduced Ferraro , presented her with a Boston Celtics T-shirt. Senator Kennedy said he expected that the Democratic ticket, like this year's National Basketball Association champions, will come from behind to win in November.
When Ferraro, wearing the bright green T-shirt, stepped up to the microphone, she began her blast against President Reagan by criticizing him for evoking the memory of President John F. Kennedy at a campaign stop last week in Waterbury, Conn.
''I resent it when ... he pretends he has anything in common with that good man,'' she said. Elaborating, she said Kennedy sent Peace Corps volunteers to Latin America, but Reagan has sent military advisers. Kennedy signed the Nuclear Test Ban Treaty, she said, but Reagan has been the first US president since Herbert Hoover not to meet with his Soviet counterpart.
''I liked her reference to Kennedy,'' spectator Mike Flaherty said after the rally. The MBA candidate at Northeastern University, who is registered as an independent, said he disagrees with Reagan's foreign policy but will vote for him anyway because of the strong economy. ''I think bread-and-butter issues will decide it (the election) this year, not war-and-peace issues.''
Others at the rally, such as Ann Schulz of Newton, Mass., disagreed.
She said she liked the strong stand for peace in Ferraro's speech, ''especially the part about sending in the diplomats before sending in the soldiers.'' Mrs. Schulz, who brought along her eighth-grade daughter, said Reagan ''could have used diplomacy more in Lebanon'' and scored him for sending the marines to Grenada ''strictly for show-of-force purposes.''
Though the day was gray and cold, spectators - forming a sea of bright red, yellow, and green rain slickers - stood elbow to elbow and waited patiently for Ferraro to arrive. Mondale-Ferraro campaign workers said buttons and bumper stickers were selling like hotcakes both before and after the rally. One campaign worker, a member of the National Organization for Women, said the turnout for Ferraro here was a triumph. But, she said, ''If she doesn't do well in Boston....'' The sentence trailed off before she could add, ''Where can she do well?''