Hillary Clinton, who lost out to Mr Trump in the 2016 USA election, said the women's marches were "a testament to the power and resilience of women everywhere".
The biggest demonstrations were taking place in Washington, New York, Los Angeles and Chicago - but there were also marches in about 250 other cities and towns across the country. Last year's L.A. march was also the largest. Organizers said they're fighting for basic rights for women, immigrants and others who are under attack.
In Park City, Utah, where the annual Sundance Film Festival is in full swing, actress Jane Fonda and nationally known attorney Gloria Allred joined the women's march.
Thousands of demonstrators gathered Saturday on the steps of the Lincoln Memorial and lined the frozen-over Reflecting Pool to rally for women's rights, urge women to run for public office and call on citizens to fully engage on issues from sexual assault and racial equality to immigrant protections and gun violence.
New Jersey's first lady Tammy Murphy told a crowd of thousands at a Women's March event in Morristown that she had been sexually assaulted while in college.
Portman said she expressed herself as "prudish, conservative, nerdy and serious" to be heard and feel safe.
"Today I will add my voice to this growing chorus". We are not here to suppress our perspectives for your relief.
Actress Viola Davis speaks at the 2018 Women's March in California.
Portman recalled how her breakout role at age 12 in the 1994 film "The Professional" led to an uncomfortable focus being put on her sexuality, including receiving a "fan mail" letter that described a rape fantasy, film reviewers discussing her breast size and a radio station doing a public countdown to her 18th birthday when an adult could legally have sex with her.More news: Russia: Iran deal can't survive if USA pulls out
"We fall asleep when we're moving ahead and we don't look to the left and right and see that we're not including people in this move-ahead", she said (via ET).
Demonstrators also rallied in favor of the Black Lives Matter and #MeToo movements.
"I think people were surprised that in a very small, isolated rural community that we were concerned enough about what was going on in the world ... that we felt it was important enough to make a stand", said resident Gwen Wilson.
Many women wore knitted pink "pussy" hats, while placards with messages such as "Grab patriarchy by the balls" and "Nevertheless she persisted" dotted the crowd. Melanie Hunter, an African- American woman from Koreatown, told City News Service while wearing a shirt that read, "Why be racist, sexist, homophobic or transphobic when you can just be quiet?"
"Trump has called on people to underscore on economic successes since he became the President, in a tweet acknowledging the marches, which were planned for the one-year anniversary of Trump entering office and aimed primarily at protesting his rhetoric and policies". Shannon was holding a sign that read "GOP = Greedy Old Perverts".
"Last year, I just felt kind of angry and impassioned", Ann Dee Allen of Wauwatosa, Wis., said at the NY march on Saturday.
The federal contractor said her community went into high gear a year ago.
Eric Garcetti, mayor of Los Angeles, wrote on his official Twitter account: "Today, we bore witness to the testimonials of incredible women at the #WomensMarch2018".