Feminist groups were joined outside Belfast City Hall by people supporting gay rights, immigrants and the disabled.
Organisers said the aim was to highlight women's rights.
The protest comes after a series of controversial comments by President Trump during his election campaign.
Protesters said they believe the rights of many Americans are under threat from the new administration.
They said they were overwhelmed by the number of people who took part.
The events draw inspiration from the Women's March on Washington, which also took place on Saturday and attracted crowds of 200,000 people.
Almost 700 so-called sister marches took place on Saturday across the globe. Protests by women have already taken place in Australia, New Zealand and Japan.
In the UK, rallies took place in London, Cardiff, Edinburgh, Lancaster, Leeds, Liverpool, Manchester, Bristol and Shipley.
London organisers announced on stage that between 80,000 and 100,000 men, women and children had taken part in the rally.
However, police have not given an independent estimate and the BBC has no way of verifying the figure.
Hundreds gathered for a Women's March in Belfast on Saturday as part of an international protest on the first day of Donald Trump's presidency.