President Donald Trump late Saturday skipped the annual White House Correspondents' Dinner and accused mainstream U.S. media of unfair criticism as he kept "one promise after another" during his first 100 days in office.

"I could not possibly be more thrilled than to be more than 100 miles away from Washington's swamp," Trump told his supporters in Pennsylvania.

Trump accused the media of being "fake news" and "out of touch", and said they should be given a "big, fat, failing grade".

The president described his first 100 days as "very exciting and very productive".

Meanwhile in Washington, D.C., famous investigative journalist Bob Woodward told participants of the White House Correspondents' Dinner: "The effort today to get this best obtainable version of the truth is largely made in good faith.

"Mr. President, the media is not fake news," Woodward said to a solemn applause from the gathering.

Headlining the event -- which Trump became the first president to miss in more than three decades -- comedian Hasan Minhaj said: "We gotta address the elephant that's not in the room. The leader of our country is not here. And that's because he lives in Moscow, it is a very long flight.

"As for the other guy, I think he's in Pennsylvania because he can't take a joke," Minhaj said.

"A large group of Hollywood actors and Washington media are consoling each other in a hotel ballroom in our nation's capital right now," Trump said. He added: "And I could not possibly be more thrilled than to be more than 100 miles away from Washington's swamp, spending my evening with all of you and with a much, much larger crowd and much better people, right?"

Earlier in the day, tens of thousands of protesters marched in D.C. in protest of the administration's climate policies. They surrounded the White House with Trump inside, chanting: "Resistance is here to stay, welcome to your 100th day."

Trump became the first president since Ronald Reagan in 1981 to skip the event — and Reagan was recovering from an assassination attempt.

The official WHCA dinner began in 1921. In recent decades, the event offered Washington's press corps an opportunity to wear black tie and stunning gowns while mixing with celebrity guests. Most people trace that development to 1987, when Baltimore Sun reporter Michael Kelly brought Fawn Hall, the secretary at the center of the Iran-Contra affair.