There was no official crowd count for the largest march, in Washington, but organizers told newsmen they were confident it drew one million people
"Watched protests yesterday but was under the impression that we just had an election! Why didn't these people vote? Celebs hurt cause badly," Trump tweeted.
A second, more formally worded, missive followed an hour later in which the Republican leader stressed that he respected the right to demonstrate.
"Peaceful protests are a hallmark of our democracy. Even if I don't always agree, I recognize the rights of people to express their views," he tweeted.
There was no official crowd count for the largest march, in Washington, but organizers told AFP they were confident it drew one million people, protesting any rollback of the rights of women, immigrants and minorities under Trump's presidency.
At least a million people marched in other US cities, according to a tally of official counts for 15 major demonstrations compiled by CNN.
That figure did not include the many smaller sister marches held across the country and worldwide, which organizers numbered at over 600.
The scale of the mass protests highlights the depth of resistance to the former reality TV star, two days after he was sworn in as president with an approval rating of just 37 percent.
Pop diva Madonna made an unannounced appearance Saturday in Washington, joining hundreds of thousands of protesters who rallied for women's rights in defiance of Trump.
Other celebrities at the massive demonstration included actresses Scarlett Johansson, Ashley Judd and America Ferrera, filmmaker Michael Moore and the feminist icon Gloria Steinem.