On the 20^th anniversary of the worst attacks in American history, US President Joe Biden was joined by former presidents Barack Obama and Bill Clinton at the September 11 Memorial in Lower Manhattan on Saturday.

Mr Biden and first lady Jill Biden will visit all three sites of the attacks during a day of somber reflection across the US, where the lives of the 2,977 victims from the Al Qaeda attacks are being remembered.

A similar number of first responders who helped clean up the toxic remnants of the collapsed Twin Towers are thought to have died from cancers, lung diseases and other health issues stemming from their work in the weeks after 9/11.

Saturday’s ceremonies will begin with a moment of silence at 8:46 am local time to commemorate the moment when the first plane, American Airlines Flight 11, hit the North Tower.

Family and friends of the victims will read the names of the nearly 3,000 loved ones who died in the attacks, as well as during the 1993 bombing of the World Trade Centre complex.

In the weeks 9/11, Americans came together in shock and mourning as countries the world over rallied behind the US in solidarity.

But 20 years on, America is more fractured than any time in its modern history and instead of presiding over a rare moment of national unity, Mr Biden faces a country grappling with the loss of the Afghanistan war and a seemingly unbeatable pandemic that has killed more than 650,000 people in the US.

In a recorded address released on Friday by the White House, Mr Biden appealed to Americans to find common ground.

“No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago,” Mr Biden said.

“In the days that followed September 11, 2001, we saw heroism everywhere, in places expected and unexpected. We also saw something all too rare: a true sense of national unity.”

After their visit to Ground Zero, as the site of the collapsed towers became known, the Bidens will take part in ceremonies in Shanksville, Pennsylvania, the site where a hijacked plane crashed; and in Arlington, Virginia, where the Pentagon was struck.

Other ceremonies will be held the length and breadth of the US, including in Guantanamo Bay, where five men accused of assisting the 9/11 hijackers are still awaiting trial.

Troops stationed on the US base carved from a tip of Cuba will go on a 9.11-kilometre remembrance run, and a special service will be held in the chapel.

Commemoration events are being held around the world on Saturday for the anniversary of September 11, including several in London, where Mayor Sadiq Khan remembered the 67 Britons killed in the attacks.

“Our values of freedom, tolerance and respect will always, always prevail. Hate will never win,” Mr Khan said.