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.
Mr Trump did not mention the protests during a bridge-building visit to the CIA headquarters in Langley, Virginia, on Saturday but instead turned on the press.
He accused the media of inventing a feud between him and the intelligence community and he called reporters "among the most dishonest human beings on earth".
Mr Trump said TV footage and photos of his inauguration had painted an inaccurate picture.
"It looked like a million and a half people" there on Friday, he said, rubbishing media reports that there were as few as 250,000 people.
He also said the crowd extended all the way back to the Washington Monument, although this claim is contradicted by aerial shots from the day.
Later, White House press secretary Sean Spicer berated reporters at a news conference over photographs that had shown large, empty spaces during the ceremony.
"This was the largest audience ever to witness an inauguration, period. Both in person and around the globe," he said in a fiery statement.
"These attempts to lessen the enthusiasm about the inauguration are shameful and wrong."
In addition to the photographic evidence, Washington's Metro system said trips were down on previous inaugurations. Marketing firm Nielsen said television views in the US were less than Barack Obama's and Ronald Reagan's first inaugurations.
Mr Spicer, who did not take questions, added: "There's been a lot of talk in the media about the responsibility to hold Donald Trump accountable, and I'm here to tell you it goes two ways. We're going to hold the press accountable as well."
Outgoing CIA chief John Brennan accused Mr Trump of "a despicable display of self-aggrandisement" over the statement at Langley.
"Former CIA Director Brennan is deeply saddened and angered at Donald Trump's despicable display of self-aggrandisement in front of CIA's Memorial Wall of agency heroes," his former deputy, Nick Shapiro, said in a statement carried by CNN.
"Brennan says that Trump should be ashamed of himself."
Last week, Mr Brennan called on Mr Trump to be more "disciplined" in what he said and warned him not to underestimate Russian intentions.
Mr Trump's visit had sought to mend relations with the intelligence community after weeks of doubting their conclusions about alleged Russian interference into the US election.
"I love you, I respect you," he said, adding that he was "1,000%" behind the spy agency.
Mr Trump said the media had invented a feud between them, although in a recent row over a leaked dossier that alleged the Kremlin held compromising material on him, he had likened the actions of intelligence agencies to Nazi Germany.
Mr Trump's election has divided opinion in the US and around the world.
German Foreign Minister Frank-Walter Steinmeier, writing in Bild newspaper on Sunday, warned that the world was headed "for turbulent times."
"With the election of Donald Trump, the world of the 20th century has definitely been overtaken," he said.
President Donald Trump has accused the media of dishonestly reporting the size of the crowd at his inauguration.