Biden, among scores of dignitaries and royals from Europe, Asia, Africa and the Americas who have arrived in London for Monday's funeral, said the queen's death at 96 years of age had left a giant hole on the global stage.
He said he had consoled the queen's heir, King Charles, that the queen would be "with him every step of the way, every minute, every moment, and that's a reassuring notion."
Hundreds of thousands of people have flooded London to bid farewell to Britain's longest-reigning monarch, with people from all walks of life from around the country and overseas queuing for hours to file past her coffin in a solemn stream.
Biden followed other world leaders in appearing on a balcony overlooking the coffin in the vast, historic Westminster Hall, making a sign of the cross and briefly placing his hand on his heart in reverence.
The U.S. president later joined Charles and other leaders for an evening reception ahead of the state funeral.