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.
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
Bruce Springsteen performed following the second moment of silence at the World Trade Center memorial service.
His song "I'll See you in My Dreams," was dedicated to all of the lives lost that day.
A second moment of silence was held at 9:03 a.m. to mark the time the south tower was hit.
A bell tolled at the World Trade Center memorial as well as other locations across the country.
