The key focus of the day is the Women's March on Washington, which organizers say could attract a quarter of a million participants.
The march, which began with a modest Facebook call in the aftermath of the November election, has grown into what could be one of the largest political demonstrations ever seen in the US capital.
But there are also more than 600 "sister marches" planned around the United States, with some of the biggest expected in Boston, New York, Chicago and Los Angeles.
And women and men in cities around the world -- including Sydney, Berlin, London, Paris and Cape Town, South Africa -- are also marching in solidarity and in opposition to the values they think Trump represents.
Australia was the scene of the first major international march, with thousands joining an anti-Trump protest in downtown Sydney. Organizers said up to 5,000 people attended the protest at Martin Place; police estimated the number was closer to 3,000.
Chants from the crowd included "women united will never be defeated" and "when women's rights are under attack, what do we do, stand up, fight back." Some carried banners with messages such as "Girl Power vs. Trump Tower" and "Dump the Trump."
Protest organizers in New Zealand's capital, Wellington, said about 700 people turned out there for a women's march. Marches were also held in Auckland, Christchurch and Dunedin.
Big crowds turned out Saturday in dozens of cities across Europe, with marchers including men, women and children, CNN reported.
Protesters who gathered outside one of Rome's most famous structures, the Pantheon, on Saturday morning carried signs such as "Yes we must" and "Women's rights are human rights."
Demonstrators also took to the streets of Berlin, Frankfurt, Munich and other cities in Germany.
Many of those taking part are concerned about Trump's agenda, his past remarks that appeared to demean women and allegations against him of sexual misconduct -- which he has denied.