Saturday’s coronation of King Charles III will be the most public display in seven decades of the curious relationship in the UK between England’s established church and the hereditary monarchy.

The King will be accompanied to and from the ceremony by thousands of military personnel, a reminder that Britain’s constitutional monarch remains head of state. But his crowning will take place within a service of communion — the sharing of bread and wine commemorating for Christians the death of Christ.

Some of the regalia used during the ceremony link back to the succession of 62 kings and queens of England and then Britain that preceded Charles III over 1,200 years. Much of the content of the ceremony also stems from traditions dating back centuries.

But novel elements will be added, and in a bid to reflect the changing demographics of faith in the UK, the Archbishop of Canterbury, Justin Welby, will preface the coronation oath with a pledge to “foster an environment in which people of all faiths and beliefs may live freely”. Leaders of other faiths and Christian denominations will be involved in parts of the ceremony, and female clergy will be given a prominent role.

More controversially, the service will include for the first time an invitation to the public to swear allegiance to the new King from their homes — a move apparently meant to encourage people to share in the event but that has instead prompted a backlash. Traditionally it was only hereditary peers who pledged loyalty to the monarch.

Jonathan Dimbleby, the broadcaster and friend of the King, blamed the archbishop’s palace for what he described on BBC radio as a “well-intentioned if rather ill-advised” initiative, that he said the King, “who never wanted to be revered”, would find “abhorrent”.

In a different spirit of inclusion and another first, the ceremony will include some words in the Celtic languages of Wales, Scotland and Ireland — a nod to ongoing strains in the union.

Formal celebrations begin with a procession from Buckingham Palace to Westminster Abbey. The King and Queen will travel in the Diamond Jubilee State Coach, built in Australia in 2012 with an aluminium frame and air-conditioned interior.

The Gold State Coach, used for every other coronation since 1831 and which will carry the royal couple back, has famously poor suspension and is “horrible” to ride in according to its last occupant, Queen Elizabeth.

At two hours, the service will be an hour shorter than the last one in 1953. During elaborately choreographed moments, the King will be bedecked with relics — sceptre, robes, orb and crowns, between them carrying thousands of jewels. The UK’s royal family is the only one in Europe that still uses such artefacts during coronations to symbolise aspects of the monarchy and its different responsibilities.

King Charles will be enthroned on St Edward’s chair, the core of which was made in 1300 to house the Stone of Scone — a sandstone block originally used during the crowning of Scottish kings. The stone was stolen by King Edward’s forces in 1296, used in London for coronations thereafter and returned to Edinburgh only in 1996. It will be on loan for the occasion.

In the part of the ceremony considered sacred and symbolic of the new King’s supposed divine right to rule, Charles will be anointed behind a screen with holy oil consecrated in Jerusalem.

There will be 2,000 invitees in attendance. Among the guests will be 850 selected community representatives.

As well as members of the extended royal family (including self-exiled Prince Harry, but not his wife Meghan Markle) world leaders expected to attend include French president Emmanuel Macron, Polish president Andrzej Duda and Anthony Albanese, the Australian prime minister. Pakistan’s prime minister Shehbaz Sharif, Philippines’ president Ferdinand Marcos Jr and European Commission president Ursula von der Leyen are also reportedly coming.

The US president Joe Biden is sending his wife, first lady Jill Biden in his place. Among other standout guests will be Michelle O’Neill, the republican leader of Sinn Féin in Northern Ireland and first minister-in-waiting. The party was previously the political wing of the Irish Republican Army, which assassinated the King’s great-uncle, Lord Mountbatten, in 1979.

China is sending vice-president Han Zheng, who oversaw the crackdown on pro-democracy campaigners in Hong Kong. The last British governor of Hong Kong, Chris Patten, said Beijing’s decision to send him was a “stick in the eye” for 140,000 Hongkongers exiled in the UK and was revealing of how the Chinese Communist party sees the country.

London’s Metropolitan Police are expecting hundreds of thousands of visitors to flock to the capital for the occasion, and will undertake the “largest one-day mobilisation in decades with just over 11,500 officers on duty”. They will be deploying specialist forces and using facial recognition technology to screen crowds for potential troublemakers.

Armed with new powers to police protests rushed into law this week, the Met warned that it would deal “robustly with anyone intent on undermining this celebration”.

A two-hour concert at Windsor Castle on Sunday evening will bring some celebrity stardust to the weekend. Thousands of street parties and other events have been planned across the UK, with the King encouraging people to bake quiche for the occasion. On Monday, the public has been invited to join in activities with local voluntary organisations in an event called “The Big Help Out”.