The Archbishop of Canterbury has paid tribute to the Queen's 70 years on the throne as a "symbol of stability and hope".
Sunday marks Accession Day and the Queen's Platinum Jubilee - the first for a British monarch.
The Queen, 95, acceded the throne after the death of her father, King George VI, on 6 February 1952.
She was radiant as she kicked off a year of celebrations to mark the milestone anniversary with a reception and cake-cutting at Sandringham.
She said she looked forward "to continuing to serve you with all my heart", and spoke of her hope that the Jubilee would "bring together families and friends, neighbours and communities after some difficult times for so many of us".
"As I look ahead with a sense of hope and optimism to the year of my Platinum Jubilee, I am reminded of how much we can be thankful for," the Queen said.
"These last seven decades have seen extraordinary progress socially, technologically and culturally that have benefitted us all; and I am confident that the future will offer similar opportunities to us and especially to the younger generations in the United Kingdom and throughout the Commonwealth."
In a statement, she added that it was her "sincere wish" that the Duchess of Cornwall become Queen Consort when Prince Charles becomes king, and thanked people "of all nationalities, faiths and ages" for their "goodwill" through the years.
Archbishop Justin Welby invited Christians to pray that God would "continue to strengthen and guide" the Queen, and "bless her with continued health and wisdom".
"Her Majesty The Queen's seventy years of service as monarch have been a symbol of stability and hope throughout the United Kingdom, the Commonwealth and the world," he said.
"As we mark the anniversary of Her Majesty's accession to the throne, we give thanks for her dedication to us all, and her faithful witness to Jesus Christ."