The public pledge is one of several striking changes to the ancient ceremony revealed on Saturday.
In a coronation full of firsts, female clergy will play a prominent role, and the King himself will pray out loud.
The Christian service will also see religious leaders from other faiths have an active part for the first time.
The Coronation on Saturday will be the first to incorporate other languages spoken in Britain, with a hymn set to be sung in Welsh, Scottish Gaelic and Irish Gaelic.
Despite changes designed to reflect other faiths, the three oaths the King will take and form the heart of the service remain unchanged, including the promise to maintain "the Protestant Reformed Religion".
Full details of the Westminster Abbey service - the theme of which is "called to serve" - have been published by Lambeth Palace.
The Archbishop of Canterbury said it would "recognise and celebrate tradition" as well as contain "new elements that reflect the diversity of our contemporary society".
The public will be given an active role in the ceremony for the first time, with people around the world set to be asked to cry out and swear allegiance to the King.
This "homage of the people" replaces the traditional "homage of peers" where hereditary peers swear allegiance to the new monarch. Instead everyone in the Abbey and watching at home will be invited to pay homage in what Lambeth Palace described as a "chorus of millions".
The order of service will read: "All who so desire, in the Abbey, and elsewhere, say together: I swear that I will pay true allegiance to Your Majesty, and to your heirs and successors according to law. So help me God."
It will be followed by the playing of a fanfare.
The Archbishop of Canterbury will then proclaim "God save the King", with all asked to respond: "God save King Charles. Long live King Charles. May the King live forever."
A spokesman for Lambeth Palace, the archbishop's office, said: "The homage of the people is particularly exciting because that's brand new.
"That's something that we can share in because of technological advances, so not just the people in the Abbey, but people who are online, on television, who are listening, and who are gathered in parks, at big screens and churches.
"Our hope is at that point, when the Archbishop invites people to join in, that people wherever they are, if they're watching at home on their own, watching the telly, will say it out loud - this sense of a great cry around the nation and around the world of support for the King."
While the oaths - which have remained unchanged for centuries - will retain their Protestant pledge, Lambeth Palace said the Archbishop of Canterbury will "contextualise" them.
He will say beforehand that the Church of England will seek to create an environment where "people of all faiths and beliefs may live freely".
"The religious and cultural context of the 17th Century was very different to today's contemporary, multi-faith Britain," a Lambeth Palace spokesperson said. "So, for the first time there will be a preface to the Oath."
The BBC's religion editor Aleem Maqbool said over the years, there has been much speculation about whether the King would change his oaths to reflect an aspiration to protect the practice of all faiths and beliefs, though it would have been a move that would have caused consternation among some Church of England traditionalists.
He added that it may appear a neat solution to leave the oaths unchanged and have the Archbishop of Canterbury express that forward-looking sentiment, but progressives will be left wondering why the protection of the practice of all beliefs could not be part of the oral contract with the nation that the King enters into.
As part of the service Muslim, Hindu, Jewish and Sikh peers will present the King with pieces of the coronation regalia, including bracelets, the robe, the ring, and the glove.
Prime Minister Rishi Sunak, a practising Hindu, will read from the biblical book of Colossians.
The blessing will be shared by leaders of different Christian denominations for the first time, including the Catholic Cardinal Vincent Nichols.
After the religious service has ended, the King will receive a greeting by Jewish, Hindu, Sikh, Muslim and Buddhist leaders.
The move reflects Charles' deeply-held belief in promoting unity between different faiths through championing interfaith dialogue and celebrating the major religions practised in the UK.
A Lambeth Palace spokesperson described the greeting as "an unprecedented gesture that will reflect the religious diversity of the Realms of King Charles III".
The greeting will not be audible for most watching outside Westminster Abbey because the Chief Rabbi will be observing the Jewish Shabbat which prohibits the use of electricity, including microphones.
The King will pray aloud using words inspired from the hymn I vow to thee my country and from the biblical books of Galatians and Proverbs.
Female clergy will be involved in the service for the first time after the Church of England allowed women to become bishops in 2014.
The Bishop of Chelmsford, Guli Francis-Dehqani, and the Bishop of Dover, Rose Hudson-Wilkin, will administer communion alongside the archbishop.
Justin Welby said the coronation was "first and foremost an act of Christian worship".
"It is my prayer that all who share in this service, whether they are of faith or no faith, will find ancient wisdom and new hope that brings inspiration and joy," he said.
New photos of King Charles and Camilla were released this weekend