King Charles III's coronation: The biggest moments of the historic event

Princess Anne, Charles' only sister, has joined royal family members at Westminster Abbey to watch her brother's coronation.

Anne and Charles were both alive when their mother was coronated in 1953. While Charles attended the ceremony, Anne did not because she was considered too young at the time.

Anne will play a special role in Charles' coronation, leading the procession back to Buckingham Palace following the service at Westminster Abbey.

Prince Harry has arrived at Westminster Abbey, marking the first time he has appeared at a royal event since last September, when he attended the queen’s funeral.

Harry, the fifth in line to the throne, is not expected to play a role in the coronation service.

He is attending alone, without his wife Meghan, or their children, Archie and Lilibet, and is expected to depart home to California shortly after the service.

As a non-working member of the royal family, Harry is dressed in a morning suit.

He entered Westminster Abbey with his cousins, Princesses Beatrice and Eugenie.

Though every coronation is rich in tradition, Charles’ coronation will look very different from his mother’s 70 years ago.

On Saturday, 6,000 fewer people will fill the pews of Westminster Abbey for today's coronation than they did for his mother's, but it will be a much more diverse and inclusive crowd.

“This is an ancient ceremony that goes back 1,000 years, but [the royals] want to use it to showcase modern Britain today,” said ABC News royal contributor Victoria Murphy. “So we're going to see inside the Abbey, ordinary members of the public in the seats that would have been occupied by aristocrats 70 years ago, and also a lot of diversity.”

“We're going to see lots of women, which was not the case in 1953, people of different ethnicities, different religions,” she said. “They’re really putting front and center the diversity that exists in Britain today.”

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