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Donald Trump has sparked a backlash after performing a double fist pump as he arrived at a memorial service for the 9/11 terror attacks.

The US president was pictured with First Lady Melania Trump as he raised both fists in the air and clenched his jaw.

Images of Mr Trump's gesture as he left Air Force One in Pennslyvania were met with outrage on social media.

The president was due to attend a sombre remembrance at the Flight 93 memorial in Shanksville.

United Airlines Flight 93 crashed on September 11, 2001 after passengers and flight crew thwarted a hijacking by a group of terrorists, and are heralded as fallen heroes for ensuring the plane couldn't be used to hit any populated areas.

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

On the day of the attack, Mr Trump boasted on New York's Channel 9 News that, after the towers collapsed, Trump Tower at 40 Wall Street was the tallest in Manhattan.

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.”

9/11 Anniversary 2018 - In pictures 19 show all 9/11 Anniversary 2018 - In pictures 1/19 President Donald Trump and first lady Melania Trump participate in the September 11th Flight 93 Memorial Service AP 2/19 Members of the White House staff observe a moment of silence on the south lawn of the White Hous Getty Images 3/19 President Donald Trump and first lady Melania Trump participate in the September 11th Flight 93 Memorial Service AP 4/19 A girl places a flower along the north pool during a commemoration ceremony for the victim Getty Images 5/19 A flower stands along the north pool at the National September 11 Memorial Getty Images 6/19 A man holds a child during a commemoration ceremony for the victims of the September 11 Getty Images 7/19 People gather during a commemoration ceremony Getty Images 8/19 Army Reserve Sergeant Edwin Morales takes a knee in front of a picture of his late cousin Ruben Correa near one of the reflecting pools of the 9/11 Memorial EPA 9/19 A guest visits the south reflecting pool as he lays his hands on the names of victims of 9/11 Reuters 10/19 A man pauses at the North pool of the 9/11 Memorial EPA 11/19 A firefighter pauses at the South pool of the 9/11 Memorial EPA 12/19 Security personnel check the area with binoculars on the roof of the Pentagon prior to a ceremony to mark the 17th anniversary of the 9/11 terror attacks Getty Images 13/19 An American flag is unfurled down the side of the Pentagon Getty Images 14/19 The US national flag is seen draped at the Pentagon building EPA 15/19 An American flag is unfurled down the side of the Pentagon Getty Images 16/19 Reserve Sgt. Edwin Morales kneels in memory of his late friend Ruben Correa, a firefighter who died in the 9/11 attacks, Getty Images 17/19 Firefighters hold up a flag that flew at the World Trade Center at the start of a commemoration ceremony Getty Images 18/19 James McGlynn of Baldwin, N.Y., looks towards the One World Trade Center site AP 19/19 White House Chief of Staff John Kelly (C) and staff gather on the on the South Lawn of the White House for a Moment of Silence EPA 1/19 President Donald Trump and first lady Melania Trump participate in the September 11th Flight 93 Memorial Service AP 2/19 Members of the White House staff observe a moment of silence on the south lawn of the White Hous Getty Images 3/19 President Donald Trump and first lady Melania Trump participate in the September 11th Flight 93 Memorial Service AP 4/19 A girl places a flower along the north pool during a commemoration ceremony for the victim Getty Images 5/19 A flower stands along the north pool at the National September 11 Memorial Getty Images 6/19 A man holds a child during a commemoration ceremony for the victims of the September 11 Getty Images 7/19 People gather during a commemoration ceremony Getty Images 8/19 Army Reserve Sergeant Edwin Morales takes a knee in front of a picture of his late cousin Ruben Correa near one of the reflecting pools of the 9/11 Memorial EPA 9/19 A guest visits the south reflecting pool as he lays his hands on the names of victims of 9/11 Reuters 10/19 A man pauses at the North pool of the 9/11 Memorial EPA 11/19 A firefighter pauses at the South pool of the 9/11 Memorial EPA 12/19 Security personnel check the area with binoculars on the roof of the Pentagon prior to a ceremony to mark the 17th anniversary of the 9/11 terror attacks Getty Images 13/19 An American flag is unfurled down the side of the Pentagon Getty Images 14/19 The US national flag is seen draped at the Pentagon building EPA 15/19 An American flag is unfurled down the side of the Pentagon Getty Images 16/19 Reserve Sgt. Edwin Morales kneels in memory of his late friend Ruben Correa, a firefighter who died in the 9/11 attacks, Getty Images 17/19 Firefighters hold up a flag that flew at the World Trade Center at the start of a commemoration ceremony Getty Images 18/19 James McGlynn of Baldwin, N.Y., looks towards the One World Trade Center site AP 19/19 White House Chief of Staff John Kelly (C) and staff gather on the on the South Lawn of the White House for a Moment of Silence EPA

While on his way to the memorial service, the president tweeted, “17 years since September 11th!" and commented on the “GREAT job” his private counsel Rudy Giuliani did while serving as mayor of New York at the time.

Nearly 3,000 people died on September 11 when planes were also flown into New York's World Trade Center and the Pentagon in an attack planned by al-Qaida leader Osama bin Laden.

Mr Trump, a New York native making his first visit as president to the Shanksville site, was set to focus on honoring the many lives that were lost that day.

"Certainly the focus will be on remembering that horrific day and remembering the lives that were lost, and certainly honoring the individuals who were not only lost that day, but also put their lives of the line to help in that process," White House press secretary Sarah Huckabee Sanders said.

Some have criticised his rocky start to a very sombre day, having focused his twitter feed on attacking Russia's investigation of the White House earlier this morning.

But CNN reported that - after the aggressive gesture - he settled into the occasion and gave a "sober" speech at the memorial

"We grieve together for every mother and father, sister and brother, son and daughter, who was stolen from us at the Twin Towers, the Pentagon and here in this Pennsylvania field," Trump said.

"We honor their sacrifice by pledging to never flinch in the face of evil and to do whatever it takes to keep America safe."

Flight 93 was heading to San Francisco from Newark, New Jersey, when passengers stormed the plane's cockpit and sought to take control from the hijackers, crashing in a field and preventing what was thought to be another planned target in Washington.

Family members of Flight 93, some of their voices breaking, read aloud the names of the 40 passengers and crew members who died. Memorial bells tolled.

Trump and his wife, Melania, traveled to the Flight 93 National Memorial from Washington and paused for a moment of reflection while overlooking the field where the plane crashed.

"They boarded the plane as strangers and they entered eternity linked forever as true heroes," Trump said of the passengers and crew.

"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."

Additional Reporting by AP.