The Queen is hosting the Commonwealth Heads of Government Meeting at a ceremony at Buckingham Palace.
Prime Minister Theresa May thanked the Queen for her "service, dedication and constancy" to the 53 nations.
Issues under discussion at the two-day summit include ocean conservation, cyber security, trade and the Queen's successor as head of the Commonwealth.
The PM said the issue of plastic waste would be "central" to the UK's agenda at the bi-annual meeting.
May said she would be "rallying" Commonwealth countries to follow the UK's fight against marine plastics - which has included a ban on microbeads and proposals to introduce a deposit-return scheme for drinks containers in England.
"The Commonwealth is a unique organization, with a huge diversity of wildlife, environments and coastlines," she said.