U.S. President Donald Trump is striking a more unifying tone as he gets down to business on his second full day in the White House.

Speaking in the East Room of the White House during a swearing-in ceremony for 30 new "assistants to the president," Trump told his top advisers that they're in the White House not to "help ourselves" but to "devote ourselves to the national good."

"This is not about party, this is not about ideology. This is about country, our country. It's about serving the American people," he said.

Trump also held up the letter left to him by his predecessor, Barack Obama — a White House tradition. "It was really very nice of him to do that," Trump said. "We will cherish that." He said the contents will remain private.

Earlier Sunday, Trump offered a scattershot response to the sweeping post-inauguration protests against his new administration, sarcastically demeaning the public opposition and then defending demonstrators' rights a short time later.

Trump, accompanied by Vice-President Mike Pence, right, holds up a letter left for him by former president Barack Obama. Trump said he appreciated the letter and that its contents will remain private. (Andrew Harnik/Associated Press)

"Watched protests yesterday but was under the impression that we just had an election! Why didn't these people vote? Celebs hurt cause badly," tweeted Trump.

Ninety-five minutes later, he struck a more conciliatory tone.

"Peaceful protests are a hallmark of our democracy. Even if I don't always agree, I recognize the rights of people to express their views," the president tweeted.

Watched protests yesterday but was under the impression that we just had an election! Why didn't these people vote? Celebs hurt cause badly. —@realDonaldTrump

Peaceful protests are a hallmark of our democracy. Even if I don't always agree, I recognize the rights of people to express their views. —@realDonaldTrump

While Trump has said that he considers Monday his first real day in office, his discordant reaction underscored that the new president has little intention of changing the defiant approach that defined his campaign — particularly when it comes to the media and those who oppose him.

Trump team offers 'alternative facts': Conway

The duelling tweets marked his administration's first response to the more than one million people who rallied at women's marches in Washington and cities across the world. Hundreds of protesters lined the street as Trump's motorcade drove past on Saturday afternoon, with many screaming and chanting.

The Washington rally appeared to attract more people than attended Trump's inauguration on Friday, but there were no completely comparable numbers. Regional transportation officials tweeted on Sunday that 1,001,616 trips were taken on the rail system on Saturday. Metro spokesman Dan Stessel had said that on Friday, the day of Donald Trump's inauguration, just over 570,000 trips were taken on the rail system.

A combination of photos taken at the National Mall shows the crowds attending the inauguration ceremonies to swear in Donald Trump at 12:01pm, left, on Jan. 20, 2017, and former president Barack Obama on Jan. 20, 2009. (Reuters)

Even suggestions of weak enthusiasm for his inauguration clearly irked the new president.

Trump spent his first full day in office berating the media over their coverage of his inauguration, using a bridge-building visit to CIA headquarters to air grievances about "dishonest" journalists and wildly overstating the size of the crowd that gathered on the National Mall as he took the oath of office.

Trump said throngs "went all the way back to the Washington monument," despite photos and live video showing the crowd stopping well short of the landmark.

'These attempts to lessen the enthusiasm of the inauguration are shameful and wrong,' says White House press secretary Sean Spicer 1:19

In talk show interviews Sunday, advisers defended Trump's anger at journalists for correctly reporting that his inauguration drew a smaller crowd than President Barack Obama's did eight years ago, saying the Trump administration was supplying "alternative facts."

"There's no way to really quantify crowds. We all know that. You can laugh at me all you want," Kellyanne Conway told NBC's Meet The Press. She added: "I think it's actually symbolic of the way we're treated by the press."

'He's not going to release his tax returns'

Aides also made clear that Trump will not release his tax returns now that he's taken office, breaking with a decades-long tradition of transparency. Every president since 1976 has released the information.

Throughout the campaign, Trump refused to make his filings public, saying they're under audit by the Internal Revenue Service and he'd release them only once that review is complete. Tax experts and IRS Commissioner John Koskinen said such audits don't bar taxpayers from releasing returns.

Trump adviser Kellyanne Conway gets ready to go on television outside the White House on Sunday. Conway says Trump has no plans to release his tax returns. (Manuel Balce Ceneta/Associated Press)

"He's not going to release his tax returns. We litigated this all through the election. People didn't care," Conway said on ABC's This Week.

Trump's Sunday plans were to include a telephone conversation with Israeli Prime Minister Benjamin Netanyahu, who told his Cabinet that the call would cover the Israeli-Palestinian conflict, Syria, Iran and other issues and a reception for law enforcement officers and first responders who helped with his inauguration.

Sunday also was his 12th wedding anniversary.

Trump to meet with Trudeau, Pena Nieto

His chief of staff, Reince Priebus, said on Fox News Sunday that Trump would spend his first full week in office undoing some of his predecessor's agenda and planned to sign executive orders on immigration and trade.

Priebus did not provide specifics but during the campaign, Trump pledged to scuttle trade deals such as a pending Trans-Pacific Partnership agreement and overturn Obama's executive order deferring deportations for 700,000 people who were brought into the country illegally as minors.

Later in the week, he'll address congressional Republicans at their retreat in Philadelphia and meet with British Prime Minister Theresa May.

Trump also announced Sunday that he's set up meetings with the Prime Minister of Canada Justin Trudeau, and Mexican President Enrique Pena Nieto.

"We're going to start some negotiations having to do with NAFTA," he said of his meeting with Mexico, along with immigration and security at the border. Trump has promised to build a wall along the length of the southern border and insisted that Mexico will pay for it.