President Donald Trump has accused the media of dishonesty over the number of people attending his inauguration.
Mr Trump was speaking after photographs were published appearing to show more people attended the inauguration of his predecessor Barack Obama in 2009.
On Saturday, millions in the US and around the world took part in protests to highlight women's rights, which activists believe to be under threat from the new administration.
In response, President Trump tweeted on Sunday: "Watched protests yesterday but was under the impression that we just had an election!
"Why didn't these people vote? Celebs hurt cause badly."
The largest US rally was in the capital, Washington DC, which city officials estimated to be more than 500,000-strong, followed by New York with some 400,000 and hundreds of thousands elsewhere, including Chicago and Los Angeles.
Earlier, Mr Trump's press secretary said it had been "the largest audience to ever see an inauguration" even though figures he cited add up to under 750,000 people.
Sean Spicer said the new US administration would hold the media accountable..
"That's what you guys should be writing and covering, instead of sowing division about tweets and false narratives.
"The President is committed to unifying our country and that was the focus of his inaugural address, this kind of dishonesty in the media, the challenging, the bringing about our nation together, is making it more difficult."
Mr Spicer said Mr Trump received a warm welcome at the CIA's headquarters in Virginia.
Some media reports suggested there were as few as 250,000 people at the National Mall, but Mr Trump said it looked more like 1.5 million people were there.
Photographs, however, suggest there were far fewer people at yesterday's event than Barack Obama's inauguration in 2009.
Mr Spicer issued a strong warning to some media outlets.
"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 that it goes two ways, we're going to hold the press accountable as well. The American people deserve better and as long as he serves as the messenger for this incredible he will take his message directly to the American people where his focus will always be."
To support the argument, Mr Spicer outlined figures amounting to 720,000 people in the Mall, despite having asserted seconds before that "no one had numbers" for the inauguration.
He also said that the number of people taking Washington's subway system on the day had been higher than during Mr Obama's second inauguration in 2013.
In fact, there were 782,000 tickets that year, but 571,000 this year, the Washington-area transit authority says.
Mr Spicer also said that plastic sheets had been used for the first time to cover the grass which "had the effect of highlighting areas people were not standing whereas in years past the grass eliminated this visual". In fact, the grass was also covered in 2013.
He added that fences and metal detectors had had an impact on attendance, but this had also been denied by officials as being a factor.
District of Columbia officials had made preparations for an estimated 700,000 to 900,000 people.
In their reaction, major US media outlets flatly denied the claims made by the US president and his spokesman.
The New York Times, singled out by Mr Spicer, denounced "false claims" and described the statements as a "striking display of invective and grievance at the dawn of a presidency".
CNN said it did not even broadcast the spokesman's statement live. It said the press secretary had attacked the media "for accurately reporting" and went on to debunk the claims.
ABC News also goes into detail to refute the claims.
Pro-Trump Fox News reported the claims unchallenged, and highlighted Mr Spicer's condemnation of a Time Magazine reporter who incorrectly reported that a bust of civil rights hero Martin Luther King Jr. had been removed from the Oval Office. The reporter later apologised for the error.
BuzzFeed News accused Mr Spicer of lying.
Millions walk in worldwide women's marches
Millions of protesters have taken to the streets of cities in the US and around the globe to march in support of women's rights, on the first full day of Donald Trump's presidency.
The demonstrators, most wearing pink knitted hats known as "pussyhats", gathered for speeches at the National Mall outside the US Capitol Building.
The biggest US rally was in the capital Washington, which city officials estimated to be more than 500,000-strong.
This far exceeded the 200,000 that had originally been expected by organisers of the Women's March on Washington.
By most estimates, it also surpassed the crowd at Friday's presidential inauguration.
The rally was one of more than 600 being held worldwide on the president's first full day in office.
They intended to march to the new president's front gate at the White House after the speeches.
A planned march to the White House proved impossible as the entire route was filled with demonstrators. Interim DC Police Chief Peter Newsham told Associated Press: "The crowd stretches so far that there's no room left to march."
Democrats Senator Elizabeth Warren told the protestors Mr Trump and the Republican Congress were ready to ram through laws that would tilt things even more in favour of those at the top.
"Now we can whimper, or we can whine, or we can fight back. I'm here to fight back," she told the gathered crowds.
She said sexism, racism and homophobia had no place in America.
"We believe that equal means equal and that's true in the workplace, it's true in marriage, it's true in every place. We will never stop fighting to ensure equality for all of our citizens."
Ms Warren said she believes in a minimum wage, affordable education, science, and that climate change is real.
Activist filmmaker Michael Moore earlier addressed the crowd, and urged women to run for public office.
The organisers of the march in Washington DC said they had extensive security plans in place.
They said the protest was not simply about women's rights and it attracted "people of all genders, ages, races, cultures, political affiliations and backgrounds", with support from the likes of Amnesty International.
As President Donald Trump's motorcade was returning to the White House after a morning prayer service, the fleet of vehicles had to manoeuvre through throngs of protesters.
As his armoured car passed through one intersection, cars began honking at him.
The event, the brainchild of Hawaiian grandmother Teresa Shook, was intended as an outlet for women and men who consider themselves feminists to vent their frustration and anxiety over Mr Trump's victory.
The flood of people stressed the city's Metro subway system, with riders reporting enormous crowds and some end-of-line stations temporarily turning away riders when parking lots filled and platforms became too crowded.
At least one station was temporarily closed to new passengers because of the crowds backed up on the platform.
A planned march in Chicago grew so large that organizers did not attempt to parade through the streets but instead staged a rally. Police said more than 125,000 people attended.
Pam Foyster, a 58-year-old resident of Ridgway, Colorado, said Saturday's atmosphere reminded her of 1960s US protests against the Vietnam War.
"I'm 58 years old and I can't believe we are having to do this again," Foyster said in Washington DC. After the Vietnam War the push for women's rights and civil rights made her "believe anything was possible. But here we are again".
Violent protests against the businessman-turned-politician took place in the streets after the inauguration, with black-clad anti-establishment activists smashing windows, setting vehicles on fire and fighting with riot police who responded with stun grenades.
One of the groups that organized the protests sent a call to members to return to the streets after the women's march ended.
The women marching in Washington DC were set to be joined by thousands more around the world.
Marches began yesterday in New Zealand cities, with Auckland attracting more than 2000 protesters.
Similar rallies were scheduled in London, Berlin, Rome and hundreds of other cities in Europe, South America, Africa and the Middle East.
Several thousand women and men joined a rally in central Sydney, with a similar number in Melbourne.
Organisers of a London rally said between 80,000 and 100,000 people had taken part there.
A couple hundred people rallied in the Czech capital of Prague on Saturday in support of the march.
In Wenceslas Square in freezing conditions, they waved the portraits of Mr Trump and Russian leader Vladimir Putin, as well banners that read: "This is just the beginning."
Prague organiser Johanna Nejedlova said: "We are worried about the way some politicians talk, especially during the American elections."
In Copenhagen, Denmark, protesters in the march's trademark pink woollen hats met outside the US Embassy.
March participant Sherin Khankan said "an alternative to the growing hatred must be created".
At a rally in Stockholm, Sweden, organiser Lotta Kuylenstjerna said "we do not have to accept his message", in a reference to Mr Trump.
- Reuters / BBC