The Queen has called Commonwealth members "guardians of a precious flame" in a message to mark Commonwealth Day.
She attended a service at Westminster Abbey where her message was played to a congregation of dignitaries and faith leaders.
The Queen and Duke of Edinburgh were joined by the Prince of Wales, Duchess of Cornwall and the Duke and Duchess of Cambridge.
More than 1,000 children were also invited to attend the service.
In the message, which was also broadcast by radio in Commonwealth countries, the Queen hailed the organisation's continuing role, saying what its member states shared was more important now than at any point in its history.
"Not only are there tremendous rewards for this co-operation, but through dialogue we protect ourselves against the dangers that can so easily arise from a failure to talk or to see the other person's point of view," the Queen said.
"Indeed, it seems to me that now, in the second decade of the 21st Century, what we share through being members of the Commonwealth is more important and worthy of protection than perhaps at any other time in the Commonwealth's existence.
"We are guardians of a precious flame, and it is our duty not only to keep it burning brightly but to keep it replenished for the decades ahead."
The Queen said the shared history of the Commonwealth's members meant they could act on the "huge advantages of mutual co-operation and understanding" to benefit their citizens.
"As a concept that is unique in human history, the Commonwealth can only flourish if its ideas and ideals continue to be young and fresh and relevant to all generations," she said.