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US President Barack Obama spoke at an evening concert in Washington that closed out a day of memorial observances.
Obama said much had changed for Americans in the 10 years since Sept. 11, 2001.
"We can never get back the lives we lost on that day, or the Americans who made the ultimate sacrifice in the wars that followed," Obama said. "Yet today, it is worth remembering what has not changed. Our character as a nation has not changed."
Nearly 3,000 people died when four hijacked airliners were crashed into the World Trade Center in New York, the Pentagon and a Pennsylvania field.
