Biden crossed himself and put his hand on his heart as he stood with his wife Jill on a gallery overlooking the flag-draped casket in the cavernous Westminster Hall while members of the public filed by, reported AFP.
The development comes as Britain prepares for the state funeral of Queen Elizabeth II on Monday, 19 September, reports have emerged suggesting the cancellation and postponement of several other hospital appointments as well as funerals as a “mark of respect."
After witnessing the sombre scene, the US leader, Japan's Emperor Naruhito, French President Emmanuel Macron and other heads of state from around the world headed for a reception with King Charles III.
Biden, who flew in late Saturday, has said Charles's mother, who reigned for a record-breaking 70 years until her death on September 8 aged 96, "defined an era".
Australia's anti-monarchy Prime Minister Anthony Albanese, who viewed the lying-in-state and met Charles on Saturday, told Sky News Australia that the queen was "a constant reassuring presence".
There was also a private audience at Buckingham Palace for Prime Minister Jacinda Ardern of New Zealand, which like Australia and 12 other Commonwealth realms now counts Charles as its sovereign.
"You could see that it meant a huge amount (to Charles) to have seen the sheer scale and outpouring of people's love and affection for her late Majesty," she told BBC television Sunday.
But in a sign of challenges ahead for the new king, Ardern added that she expected New Zealand to ditch the UK monarchy "over the course of my lifetime".
Members of the public were already camping out in advance to catch a glimpse of Monday's grand farewell at Westminster Abbey, which is expected to bring London to a standstill and be watched by billions of viewers worldwide.
(With inputs from agencies)
