Queen Elizabeth II led celebrations of the 800th anniversary of the Magna Carta Monday, a charter demanding basic human rights -- at a time when rights legislation is under scrutiny in the United Kingdom.

The Magna Carta -- Latin for Great Charter -- was signed outside London in 1215, when tyrannical King John met disgruntled barons and agreed to a list of basic rights. It’s considered the founding document of English law and civil liberties and the beginning of modern democracy.

Prime Minister David Cameron joined the queen, the Duke of Edinburgh, Princess Anne and Prince William Monday at Runnymede in Surrey -- a riverside meadow near London where the document was originally signed -- to mark the historic day with speeches and musical performances.

Cameron took the opportunity to speak about the importance of fundamental reforms to human rights laws in the United Kingdom, Sky News reported.  

"It is our duty to safeguard the legacy, the idea, the momentous achievement of those barons. And there couldn't be a better time to reaffirm that commitment than on an anniversary like this,” Cameron told the crowd.

"Why do people set such store by Magna Carta?” Cameron asked. "Because they look to history. They see how the great charter shaped the world, for the best part of a millennium, helping to promote arguments for justice and for freedom."

"It falls to us in this generation to restore the reputation of those rights -- and their critical underpinning of our legal system," Cameron added.

But opponents accuse him of trying to undermine rights. Cameron's Conservative government wants to replace the Human Rights Act with a Bill of Rights, a move opponents fear could weaken key protections. The prime minister also withdrew Britain from the obligations of the European Convention on Human Rights.

The queen did not give a speech, but wrote in the event’s program that the Magna Carta's principles were "significant and enduring," the BBC reported.

Princess Anne also rededicated the memorial, saying the Magna Carta "provides us with one of our most basic doctrines -- that no person is above the law.”

"In recent history and even today we see in many parts of the world that power without the rule of law can lead to human suffering of terrible proportions. But it takes all of us to stand up for these principles," she said.

A replica of the Great Charter began a journey down the Thames River as part of the commemorations. The Royal Barge Gloriana led 200 boats and is due to arrive at Runnymede Monday afternoon.

Prince William also unveiled an art installation, called “The Jurors,” which highlights one of the Magna Carta's most important clauses. Twelve bronze chairs symbolize the right to a jury trial. Each chair is decorated with images relating to past and ongoing struggles for freedom, rule of law and equal rights.

It features a bullhorn belonging to gay rights campaigner Harvey Milk, the first openly gay person to be elected to public office in California.

There are only four known copies of the original Magna Carta in existence today, from an estimated 13 that were made. Two are held by the British Library, with Salisbury Cathedral and Lincoln Cathedral holding the others.

The Associated Press contributed to this report.