US President Donald Trump has described his first 100 days in office as an "incredible journey" during a speech at a rally in the state of Pennsylvania.

He used the same speech on Saturday to attack the news media and praise his own accomplishments.

Delivering the speech to supporters at the Harrisburg rally, which he had chosen to attend over the black-tie gala White House Correspondents' Dinner in Washington, DC, Trump began by launching a fresh attack on the news media.

"A large group of Hollywood actors and Washington media are consoling themselves in a hotel ballroom in our nation's capital right now," he said.

"Media outlets like CNN and MSNBC are fake news ... and they would love to be with us here tonight, but they're trapped at the [White House Correspondents'] dinner which will be very very boring."

Trump is the first president in 36 years to not attend the annual tradition at which White House officials mingle with members of the press corps and Hollywood celebrities.

Trump, however, said he was "thrilled to be more than 100 miles from Washington" to discuss the "great journey" of his first 100 days in office.

Pennsylvania was critical to Trump's victory. He won the state with 48 percent of the vote, the first time the state had voted for a Republican presidential candidate since George HW Bush in 1988.

Trump said some of his achievements during this phase of his term were the appointment of Neil Gorsuch to the Supreme Court, the decision to withdraw from the Trans-Pacific Partnership (TPP) trade deal and ending offshoring to "bring back American jobs".

"The TPP would have been a tremendous disaster for our country," he said.

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Trump also used the speech to hint that the US was planning to withdraw from the Paris climate change agreement.

"I'll be making a big decision on the Paris accord over the next two weeks, and we'll see what happens," he said.

Meanwhile, several thousand people marched from the Capitol to the White House in Washington, DC in protest against the Trump administration's energy policy.

According to the organisers, similar protests were taking place in 300 towns and cities across the country.

At the 100-day mark, opinion polls show that Trump's supporters during the campaign remain largely loyal.

Though the White House created a website lauding its accomplishments of the first 100 days, Trump has tried to play down the importance of the marker.

"It's a false standard, 100 days," Trump said while signing an executive order on Friday, "but I have to tell you, I don't think anybody has done what we've been able to do in 100 days, so we're very happy."

Trump is turning to what he is describing as the nation's biggest tax cut. It apparently falls short of Reagan's in 1981, and tax experts are sceptical that the plan would pay for itself.

The economy, so far, has been Trump's ally. Polls show that Americans feel slightly better about his job performance on that subject than his job performance overall.

The Correspondents' Dinner this year had fewer Hollywood celebrities in attendance. Television networks even invited behind-the-scenes employees.

White House reporters noted seeing more colleagues in the room than in recent years.

Organisers said they were focusing on the main purpose of the dinner, honouring journalists for their work and awarding scholarships to students.

Bob Woodward, the veteran Washington Post journalist who with Carl Bernstein broke the Watergate scandal in 1972, addressed Trump's criticism of the media.

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"Mr President: The media is not fake news," Woodward said.

Addressing fellow journalists, he said: "Whatever the climate, we should and must persist and I believe we will."

Traditionally the US president holds a humorous speech at the event in which he pokes fun at the media, politicians and even himself, and a hired comedian also mocks the president.

"We are here to celebrate the press, not the presidency," Jeff Mason, the White House Correspondents' Association president, said.