He said the crowd had reached the Washington monument as he spoke at the US Capitol, despite photographic evidence to the contrary.
Later, his White House press secretary said it had been "the largest audience to ever see an inauguration, period".
On Saturday, millions in the US and around the world protested against Mr Trump's new administration.
The largest US rally was in the capital Washington, which city officials estimated to be more than 500,000-strong. By most estimates, it surpassed the crowd at Friday's inauguration.
The aim was mainly to highlight women's rights, which activists believe to be under threat from the new administration.