"America will never, ever submit to tyranny", Trump said, noting that almost 5.5 million Americans have joined the United States military since 9/11.
Mr. Trump will be accompanied by his wife Melania on a day that has come to symbolize national unity - and which should offer him a welcome respite from the political mayhem swirling around him in Washington.
They'll remember almost 3,000 people killed when hijacked jetliners crashed into the World Trade Centre, the Pentagon and a field in Pennsylvania on 11 September 2001.
A flower is left at the North pool during a commemoration ceremony for the victims of the September 11 terrorist attacks at the National September 11 Memorial, September 11, 2017 in New York City.
President Donald Trump and first lady Melania Trump plan to join an observance at the 9/11 memorial near Shanksville, Pennsylvania.
"Free people are never at the mercy of evil", Trump told crowds at the memorial service on the crash site of Flight 93.
At the Pentagon, where 184 people died when American Airlines Flight 77 struck the building, Vice President Pence said, "A growing number of Americans have no living memory of what happened here". Trump, a Republican and native New Yorker, took the occasion of last years anniversary to issue a stern warning to extremists that "America can not be intimidated".
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The 9/11 plane strikes, which destroyed the Twin Towers and left a gaping hole in the Pentagon, killed almost 3,000 people, majority in Manhattan.
In December, a would-be suicide bomber set off a pipe bomb in a subway passageway near Times Square, authorities said.
"When they learned that, it galvanized them as a group", Clark said.
Trump has said when talking about Muslims that "thousands of people were cheering" in Jersey City, New Jersey, across the Hudson River from lower Manhattan, as the towers collapsed.
"You have to move on", she said. Researchers have documented elevated rates of respiratory ailments, post-traumatic stress disorder and other illnesses among people who spent time in the rubble.
Meanwhile, rebuilding continues. A subway station destroyed on 9/11 finally reopened Saturday.
A man looks at the north reflecting pool, which sits in the footprint where the north tower of the World Trade Center once stood.