President Obama has urged Americans to stay united and protect diversity on the 15th anniversary of the 9/11 attacks. At ground zero in New York, the names of those killed were read out in keeping with tradition.
Officials and relatives of 9/11 victims gathered at various sites across the country on Sunday to mark the terror attacks that killed almost 3,000 people 15 years ago.
Speaking at the memorial service in the Pentagon near Washington DC, US President Barack Obama praised the resilience of the victims' families.
He described the anniversary as a "difficult" day, but added that it "reveals the love and faithfulness in your hearts and in the heart of our nation."
The leader also urged Americans not to let their enemies divide them.
"Our diversity, our patchwork heritage, is not a weakness. It is still and always will be one of our greatest strengths," Obama said in his Pentagon speech.
"This is the America that was attacked that September morning. This is the America that we must remain true to."
The US president also laid a wreath at the site where 184 people lost their lives when a hijacked plane crashed into the building in 2001.
'It doesn't get easier'
The main event was held at ground zero in New York City, with hundreds of relatives of 9/11 victims attending the commemoration. The ceremony started at 8:46 local time, marking the moment when the first plane hit the north tower of the World Trade Center. The local places of worship sounded their bells to signal the start of event.
After the moment of silence, officials started reading through the complete list of 9/11 victims, most of them employees at the destroyed Twin Towers. Hundreds of the city's police and firefighters also died while trying to save civilians from the burning buildings.
"It doesn't get easier. The grief never goes away. You don't move forward - it always stays with you," Tom Acquaviva, who lost his son Paul, said at the site.
Presidential candidates Hillary Clinton and Donald Trump attended the ceremony, but were not expected to speak to reporters. They also halted their TV campaign ads in accordance with tradition.
Clinton left the event early after feeling "overheated" and briefly stayed in the apartment belonging to her daughter Chelsea. Upon leaving the Manhattan building a few hours later, the 69-year-old presidential candidate smiled and posed for a picture with a young girl.
"It's a beautiful day in New York. I'm feeling great," she told reporters.
The incident is likely to fuel speculation about Clinton's health, which some of her critics have already brought up during the campaign. She has dismissed it as "conspiracy theories."
Mourning in Pennsylvania
The local churches rang bells five more times during the read, commemorating the moment when the other plane slammed into the south tower, the moment of impact at the Pentagon, and the crash of Flight 93 in Pennsylvania after passengers rebelled against the hijackers. The last two tolls marked the moments the two towers collapsed.
A ceremony was also held near the crash site in Shanksville, Pennsylvania, where a visitor center was opened last year to commemorate the 40 passengers and crew who were killed when the United Airlines flight came down.
dj/tj (Reuters, AP, AFP, dpa)