The anniversary was marked by several events across the country.
Saturday marks 20 years since the 9/11 terrorist attacks.
Hijackers crashed two commercial airliners into the World Trade Center, striking the north tower at 8:46 a.m. followed by the south tower at 9:03 a.m. At 9:37 a.m., a third hijacked airline crashed into the Pentagon.
Twenty-two minutes later, the World Trade Center's south tower collapsed. A fourth hijacked plane crashed into a field in rural Shanksville, Pennsylvania, at 10:03 a.m. after passengers fought with and overcame the hijackers.
At 10:28 a.m. the World trade Center's north tower collapsed.
In total, 2,977 people were killed, including many New York City first responders.
The anniversary will be marked by several events across the country, including the annual commemoration at the World Trade Center Memorial in downtown Manhattan.
A fourth moment of silence took place at 9:59 a.m., the time when the south tower of the World Trade Center collapsed.
Bells tolls at the at the National September 11th Memorial & Museum and at locations across the city to mark this moment.
Defense Secretary Lloyd Austin spoke at the Sept. 11 memorial at the Pentagon, honoring the 184 people killed in the building and aboard Flight 77.
"Today, we remember not just who our fallen teammates were, but the mission that they shared. And we recall their common commitment to defend our republic, and to squarely face new dangers," he said.
Austin talked about the members of the military who were born after the attacks and the sacrifices they've made to keep the nation safe, including the 13 service members who were killed in Afghanistan last month.
"It is our responsibility to remember, and it is our duty to defend our democracy," he said.
-ABC News' Matt Seyler
At 9:37 a.m. a third moment of silence was held to mark the time when Flight 77 struck the Pentagon.
"Taps" played outside the Pentagon, while at the World Trade Center, a bell tolled.
Leaders from around the world shared their reflections and condolences on the 20th anniversary of the Sept. 11 attacks.
Queen Elizabeth said in a statement she was still moved by the unity shown by the world during such a tragic time.
"It reminds me that as we honour those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild," she said in a statement.
The U.S. national anthem played at the changing of the guard at Windsor Castle.
Ursula von der Leyen, the president of the European Union Commission, tweeted, "The EU stands with the USA in defending freedom & compassion over hate."
UN Secretary General Antonio Guterres issued a statement honoring first responders and that his "thoughts are with the victims and their families."
-ABC News' Guy Davies
