Chanting "we've only just begun" demonstrators said the law put the lives of women in danger and refused them the right to control their own bodies.
Women's reproductive rights defenders gathered in the capital, Montgomery, and in Birmingham, Anniston and Huntsville, to denounce the "Alabama Human Life Protection Act," or HB314, which virtually outlaws terminations.
"People should have the right to make the decisions that are best for their bodies without state interference," organizers said on Facebook.
The Alabama law would take effect in six months. Legislation to restrict abortion rights has been introduced this year in 16 states, four of whose governors have signed bills banning abortion if an embryonic heartbeat can be detected.
"We're going to return to the back alleys. We're going to return to where women will do abortions to themselves," 81-year-old Maralyn Mosley told the Montgomery Advertiser.
She had an abortion at 13, after her uncle raped her.
"We will return to the coat hangers and perforated uteruses. We will return to where women will bleed to death," she warned.
Sunday's rallies follow protests last week that saw women donning the iconic red tunics and white bonnets worn by the oppressed women of a dystopian future America in Hulu's "The Handmaid's Tale."
The Alabama law is likely to be blocked in state courts before its November launch date but Republican Governor Kay Ivey acknowledged when she signed it that it was part of as a wider Republican offensive to get the issue relitigated on the national stage.