Donald Trump has sparked fresh controversy by greeting supporters with a double fist-pump gesture while arriving in Pennsylvania ahead of a memorial service for the September 11th attacks.

The commander-in-chief can be seen in the photo with First Lady Melania Trump, as he’s raising both fists in the air and clenching his jaw. Mr Trump was on his way to pay respects for the fallen heroes of United Airlines Flight 93. The plane crashed as passengers and flight crew thwarted a hijacking by a group of terrorists.

The picture was shared by reporters instantly, quickly going viral across social media as the president’s critics condemned his conduct.

Donald Trump is seen arriving in Johnstown, Pennsylvania while on his way to a 9/11 tribute (REUTERS)

In a statement ahead of Mr Trump’s Tuesday speech, White House Press Secretary Sarah Huckabee Sanders said the president’s focus for the day was “remembering the lives that were lost, and certainly honouring the individuals who were not only lost that day, but also put their lives on the line to help in that process”.

While on his way to the memorial service, the president tweeted, “17 years since September 11th!”

He also tweeted about the “GREAT job” his private counsel Rudy Giuliani did while serving as mayor of New York City during the attacks 17 years ago. “Rudy is a TRUE WARRIOR,” Mr Trump wrote.

Mr Trump observed the solemn anniversary for the first time as president last year. He and the first lady led a moment of silence at the White House accompanied by aides and administration officials at the exact time that hijackers flew the first of two planes into the World Trade Centre’s Twin Towers.

Nearly 3,000 people died on September 11 when other planes were flown into New York’s World Trade Centre and the Pentagon in an attack planned by Al-Qaeda leader Osama bin Laden. Nearly a decade later, bin Laden was killed in May 2011 during a US military operation ordered by President Barack Obama.

Donald Trump says he has 'tallest building' in downtown Manhattan hours after 9/11 attack

On Tuesday, the president declared the 17th anniversary “Patriot Day 2018”.

“On Patriot Day, we honour the memories of the nearly 3,000 precious lives we lost on September 11, 2001, and every hero who has given their life since that day to protect our safety and our freedom,” the proclamation read. “We come together, today, to recall this timeless truth: When America is united, no force on earth can break us apart. Our values endure; our people thrive; our nation prevails; and the memory of our loved ones never fades.”

The president was in his Manhattan penthouse not far from the Twin Towers during the attacks.

Mr Trump has said when talking about Muslims that “thousands of people were cheering” in Jersey City, New Jersey, situated across the Hudson River from lower Manhattan, as the towers collapsed. There is no evidence in news archives of mass celebrations there by Muslims.

The president has also said he lost “hundreds of friends” in the attack on New York City. He has not provided any names but has mentioned knowing a Roman Catholic priest who died while serving as a chaplain to the city’s fire department.

On Tuesday, Mr Trump reflected on the attacks in prepared remarks at the Pennsylvania field, where 44 people were killed, including 4 hijackers.

“This field is now a monument to American defiance. This memorial is now a message to the world: America will never, ever submit to tyranny,” the president said in his remarks at the memorial event. "We honour their sacrifice by pledging to never flinch in the face of evil and to do whatever it takes to keep America safe."