People protested the leaked draft decision by the Supreme Court, which indicates the court is on the brink of overturning Roe v. Wade.
Supporters rallied at City Hall and around King of Prussia, Montgomery County, chanting the theme: "bans off our bodies."
Activists spoke of the need to mobilize quickly because Republican-led states are poised to enact tighter restrictions.
In the nation's capital, thousands gathered in drizzly weather at the Washington Monument to listen to fiery speeches before marching to the Supreme Court, which was surrounded by two layers of security fences.
The mood was one of anger and defiance, three days after the Senate failed to muster enough votes to codify Roe v. Wade.
"I can't believe that at my age, I'm still having to protest over this," said Samantha Rivers, a 64-year-old federal government employee who is preparing for a state-by-state battle over abortion rights.
A half-dozen anti-abortion demonstrators sent out a countering message, with Jonathan Darnel shouting into a microphone, "Abortion is not health care, folks, because pregnancy is not an illness."
Polls show that most Americans want to preserve access to abortion - at least in the earlier stages of pregnancy - but the Supreme Court appeared to be poised to let the states have the final say. If that happens, roughly half of states, mostly in the South and Midwest, are expected to quickly ban abortion.
The battle was personal for some who came out Saturday.
Teisha Kimmons, who traveled 80 miles to attend the Chicago rally, said she fears for women in states that are ready to ban abortion. She said she might not be alive today if she had not had a legal abortion when she was 15.
"I was already starting to self harm, and I would have rather died than have a baby," said Kimmons, a massage therapist from Rockford, Illinois.
In New York, thousands of people gathered in Brooklyn's courthouse plaza before a march across the Brooklyn Bridge to lower Manhattan for another rally.
"We're here for the women who can't be here. And for the girls who are too young to know what is ahead for them," Angela Hamlet, 60, of Manhattan, said to the backdrop of booming music.
Robin Seidon, who traveled from Montclair, New Jersey, for the rally, said the nation was a place abortion rights supporters have long feared.
"They've been nibbling at the edges, and it was always a matter of time before they thought they had enough power on the Supreme Court, which they have now," said Seidon, 65.
The upcoming high court ruling in a case from Mississippi stands to energize voters, potentially shaping the upcoming midterm elections.
In Texas, which has a strict law banning many abortions, the challenger to one of the last anti-abortion Democrats in Congress marched in San Antonio.
At many of the rallies, speakers put the issue in stark terms, saying women and girls will die if abortions are outlawed.
In Los Angeles, high-profile lawyer Gloria Allred recounted how she could not get a legal abortion after being raped at gunpoint in the 1960s. She ended up having life-threatening bleeding after a "back alley" abortion.
"I want you to vote as though your lives depend on it, because they do," she told the crowd.
The Associated Press contributed to this report.