He accused the media of "deliberately false reporting" both with regard to photos of the crowd that were published as well as crowd estimates.
"No one had numbers. Because the National Park Service, which controls the National Mall, does not put any out," Spicer said.
"These attempts to lessen the enthusiasm of the inauguration are shameful and wrong," he added.
Spicer took no questions after delivering his remarks.
Crowds at Trump’s inauguration didn’t appear to measure up to those at Barack Obama's inaugurations, according to the D.C. metro authority's initial ridership estimates:
Metro Ridership: As of 11am, 193k trips taken so far today. (11am 1/20/13 = 317k, 11am 1/20/09 = 513k, 11am 1/20/05 = 197k) #wmata
— Metro (@wmata) January 20, 2017
Spicer hit the assembled White House press corps with some different numbers, though: He claimed that 420,000 people used the D.C. metro on Trump's Inauguration Day, compared to 317,000 for Obama's 2013 inauguration.
Those numbers simply don't match up with what Metro has reported. According to the Washington Post, Metro said 570,557 people took trips on Friday in total, compared with 1.1 million trips at Obama's 2009 inauguration and 782,000 at his 2013 inauguration.
It's unclear where Spicer got the numbers he relayed to reporters Saturday evening or what measure they were based on.
Aerial photos of the crowd at Trump’s inauguration had shown much open space on the National Mall when compared side-by-side with an aerial photo of Obama’s 2009 inauguration.
Spicer told reporters in the White House briefing room that Trump's inauguration was the first to use floor coverings to protect the grass on the National Mall. He said that aerial photos framed around the area of those floor coverings were misleading because the coverings had the effect of highlighting spaces where people were not standing.
He also said that inauguration attendees were unable to enter the National Mall as quickly as in years past due to increased fencing and use of magnetometers.
Trump, too, picked on the press earlier in the afternoon in remarks at CIA headquarters in Langley, Virginia, claiming that from his vantage point the crowd at his inaugural address stretched as far back as the Washington Monument and “looked like a million and a half people.”
He said he turned on an unspecified TV network by “mistake” and heard a very different crowd estimate.
“It showed an empty field. And it said we drew 250,000 people. Now that’s not bad. But it’s a lie,” he said. “We had 250,000 people literally around, you know, in the little bowl we constructed. That was 250,000 people. The rest of the 20-block area all the way back to the Washington Monument, that was packed.”
Before addressing the media's reporting on inaugural crowd size, Spicer also took time to recount what he called a "particularly egregious" example of false reporting on Twitter. He referred to a reporter who'd tweeted that a bust of Martin Luther King, Jr. had been removed from the Oval Office.
"This was irresponsible and reckless," Spicer said.
Watch Spicer's remarks below:
This post has been updated.