Queen Elizabeth led celebrations on Monday to mark 800 years since the sealing of the Magna Carta, one of the world's most significant historical documents and credited with paving the way for modern freedoms and human rights.

On June 15, 1215, in fields by the banks of the River Thames at Runnymede to the west of London, England's King John agreed to the demands of his rebelling barons and accepted the Magna Carta, Latin for "Great Charter", which for the first time placed the monarch under the rule of law.

In the centuries since, it has taken on huge global significance, becoming the basis for the US Bill of Rights, the US Declaration of Independence and the Universal Declaration of Human Rights. Three of its 63 clauses still remain on Britain's statute book.

"What happened in these meadows eight centuries ago is as relevant today as it was then. And that relevance extends far beyond Britain," Prime Minister David Cameron said.

He said the document had changed the world, inspiring people from the founding fathers of the United States and Indian independence leader Mahatma Gandhi to Nelson Mandela in South Africa.

"Its remaining copies may be faded, but its principles shine as brightly as ever," Cameron told the ceremony attended by the queen, other royals and global figures including US Attorney General Loretta Lynch.

Magna Carta came into being during a period of great political upheaval in England with conflict between King John, his nobles and the English church.

It was essentially a peace deal to address the problems of the day and was annulled by the pope shortly afterwards. But updated versions, which included two original clauses regarded as pivotal in establishing the rule of law, were re-released regularly by or on behalf of succeeding monarchs.

The clauses read: "No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any way, nor will we proceed with force against him, or send others to do so, except by the lawful judgment of his equals or by the law of the land.

"To no one will we sell, to no one deny or delay right or justice."

ORIGINALS EXIST

Four original copies of the document, written on a single sheet of parchment about the size of A3 paper, still exist.

At Monday's ceremony, a new art installation was unveiled and the American Bar Association's Magna Carta Memorial, which was erected at the site in 1957, was re-dedicated.

US Attorney General Lynch said the charter was a bedrock to free societies globally, while Cameron also used the anniversary as a political opportunity to underpin his plan to overhaul human rights laws and reduce the influence of Europe.

However, John Dyson, chairman of the Magna Carta Trust, said King John and the barons would have been bemused that the document would garner such interest hundreds of years later.

"They would surely have been astonished that over time Magna Carta came to be regarded as one of the most important constitutional documents in our history," he said.

"They would not have believed that barons' lists of demands would become a symbol of democracy, justice, human rights and perhaps above all, the rule of law for the whole world. But that is exactly what has happened."