King Charles welcomed world leaders to Buckingham Palace on Sunday ahead of the funeral of Queen Elizabeth, as US President Joe Biden paid a final tribute to the late monarch.

Biden crossed himself and put his hand on his heart while he stood with his wife Jill on a gallery overlooking the flag-draped casket in London's cavernous Westminster Hall.

Members of the public filed by as time ticked down for them to pay their last respects to the only sovereign most Britons have ever known before she is laid to rest on Monday.

Biden said the queen, who reigned for a record-breaking 70 years until her death on September 8 aged 96, exemplified the "notion of service".

"To all the people of England, all the people of the United Kingdom, our hearts go out to you, and you were fortunate to have had her for 70 years, we all were. The world's better for her," Biden said after signing a book of condolence.

The US president then headed to Buckingham Palace for a reception hosted by Charles for the dozens of leaders from Japan's reclusive Emperor Naruhito to France's Emmanuel Macron attending the funeral. (AFP)