Charles pledges the coronation oath

Charles places his hand on the Holy Bible and says, "The things which I have here before promised, I will perform and keep. So help me God."

The king signs an oath, pledging to serve the people and rule according to law.

The coronation oath marks the only time a monarch signs a written obligation to their people, according to ABC News royalty consultant Alistair Bruce.

Following the oath, Charles kneels before the High Altar and prays. Charles’ prayer is inspired by the hymn, “I vow to thee, my country.”

During the recognition, as it is known, Charles is presented to each of the four points of the compass.

He is presented to the East by the Archbishop of Canterbury; to the West, by Christopher Finney, a holder of the George Cross; to the North by Baroness Amos, a Lady of the Order of the Garter; and to the South by Lady Eilish Angiolini, a Lady of the Order of the Thistle.

This is to affirm and acknowledge that the king is the one they recognize as head of state.

The two-hour coronation service is being led by the archbishop of Canterbury, Justin Welby, who has participated in royal weddings, funerals and christenings.

Per tradition, Welby authorized a new liturgy for Charles’ and Camilla’s coronation, which focuses on loving service to others.

All three of William and Kate’s children are attending the coronation service. Charlotte, 8, and Louis, 5, are seated with their parents inside Westminster Abbey.

Their older brother, George, second in line to the throne, is serving as a Page of Honour for Charles in the service.

Leaders of anti-monarchy campaign group Republic were arrested near Trafalgar Square as they arrived to protest Charles' coronation, ABC News has learned.

Hundreds of protesters have gathered in Trafalgar Square along the royal procession route as part of a demonstration organized by Republic, which advocates to replace the British monarchy with an elected head of state.

Demonstrators were seen wearing yellow and chanting loudly, "Not my king!" Some were waving big banners that read "Abolish the Monarchy." Others held homemade signs with slogans like "Spare a Palace."

Protesters told ABC News they believe the monarchy is out of date -- a democratic symbol of entrenched privilege and colonialism, as well as an unnecessary expense during the a cost of living crisis in Britain. One man said he would be happy if Charles ran for president.

The protesters are mingled with a larger crowd of people who came to watch the coronation -- an illustration of how those who strongly support the monarchy still outnumber those who strongly oppose, at least for now. A young woman waved a placard with a photo of Meghan, Duchess of Sussex, alongside the words "Princess of Our Hearts."

-ABC News' Ibtissem Guenfoud and Patrick Reevell