As the proclamation "long live the king" rang out in Westminster Abbey, it was a very different atmosphere in Trafalgar Square -- hundreds of protesters booed loudly and chanted "not my king."

The protesters have been there since the morning and were mixed among royals fans. Yellow banners reading "Abolish the Monarchy" waved in the air, dueling with the Union Jack flags held by people celebrating the coronation.

It's rare to see an anti-royal demonstration in the United Kingdom. The number of people who gathered in support of the crown far outnumbered those who were protesting. However, the issues that motivate the protesters, such as the monarchy being out of date and linked to colonialism, are ones that present serious challenges that Charles will immediately face in trying to keep the crown relevant.

Among those challenges that will emerge immediately are countries in the British Commonwealth that are seeking to remove Charles as head of state. Jamaica has signaled its plans to do so soon and there are questions whether Australia could follow in the coming years.

-ABC News' Patrick Reevell

As the choir sings hymns composed for past coronations, Charles and Camilla enter the Shrine of St Edward the Confessor.

As they depart, Charles is wearing the Imperial State Crown, while Camilla is wearing the St. Mary’s Crown.

The Imperial State Crown was made in 1937 for the coronation of Charles’ grandfather. It contains 2,868 diamonds, 17 sapphires, 11 emeralds, 269 pearls and four rubies.

Charles will wear the crown again on occasions like the state opening of Parliament.
The crown was last seen publicly atop the coffin of Queen Elizabeth II.

As Charles was crowned king, ABC News royal contributors weighed in on the moment.

Robert Jobson said he was feeling "deep pride in my country and the royal family and the clergy."

"Particularly a bit of relief for the king because I thought when the archbishop was trying to get that crown on his head, he was trying a little too hard," Jobson added, with a laugh.

Victoria Murphy commented on how the "mood really shifted."

"It was very solemn, it was quite tense, it felt very serious and then suddenly, it actually feels quite joyous now," Murphy said. "It feels like a moment of celebration."

Sir Peter Westmacott, who previously served as Charles' secretary, said he also felt "a sense of great pride" and is "delighted for my old boss."

"It's a wonderful moment of pageantry and the mixture of humility and service," Westmacott added, "but also the crowning of a monarch in a way that no other country in this world still does or indeed knows how to do as we do."

Ailsa Anderson, who previously served as press secretary to Charles' late mother, Queen Elizabeth II, described feeling "rather emotional about it all."

"However you prepare for something like this, to actually see it has made me swell with pride but also emotional and also proud of my former boss, the Archbishop of Canterbury, who has been faultless in this service," Anderson said. "I think this service is pretty stupendous."

Kristina Kyriacou, who previously served as the communications secretary to Charles and Camilla, said she was "absolutely overjoyed."

"I couldn't be happier. I feel very, very emotional," she added. "This is a king who has worked every second and every hour of the day for the past 60 years. You know, since his teens, he has absolutely worked in public service serving his mother, serving the nation and the commonwealth -- and he deserves this. And I hope everyone understands all the good that he has done and all the causes that he has championed."

Charles and Camilla removed the St Edwards Crown and St. Mary’s Crown, respectively, before taking their seats at the Chairs of Estate and receiving communion.

The bread and wine for the communion is consecrated with the same wording from the Book of Common Prayer as was used at the queen’s coronation in 1953.

The Greek singing underway in Westminster Abbey is a tribute to Charles’ father, Philip, who was born in Greece and was a descendant of Greek royalty.

Charles is now wearing the Sword of Offering, a symbol of the protection of good and the punishment of evil. He is also presented with the Armills, referred to as the “bracelets of sincerity and wisdom.”