The King's Coronation, the first in the UK for 70 years, is under way at the Westminster Abbey.
A military band played the national anthem as the King and Queen Consort set off from Buckingham Palace, as crowds erupted into thunderous cheers, BBC reported.
As the couple arrived at the abbey in the Diamond Jubilee State Coach, the King was seen smiling and waving.
Around 2,300 people are inside the abbey for the service, with Prince Harry arriving alongside his cousins.
Celebrity guests are also in place -- including actress Emma Thompson and US singer Katy Perry.
US First Lady Jill Biden, and her granddaughter Finnegan, arrived in a three-car motorcade. US President Joe Biden is not in the UK.
French President Emmanuel Macron and First Lady of Ukraine Olena Zelenska are also in the abbey.
Prime Minister Rishi Sunak, along with leaders of the Commonwealth countries, entered with their respective flag bearers. The service, which began at 11:00 BST, will last for around two hours, the BBC reported.
A key theme of the Coronation for King Charles will be service -- and in his first prayer when he reaches the Abbey, he will say: "I come not to be served, but to serve."
Following the ceremony, the King Charles III and Queen Camilla will return to the Buckingham Palace.
Prince Harry, the Duke of Sussex, arrived from the US on Friday on a commercial flight, and was seen entering the abbey with his cousins Princess Eugenie and Princess Beatrice.
It is the first time since he has been in public with his brother, Prince William, since his memoir, Spare, was released.
It is thought Prince Harry may fly back to the US just hours after the ceremony to rejoin his wife Meghan, as their son Archie is celebrating his fourth birthday.
On the procession route, umbrellas started to pop up as the rain arrives.
But despite the very English weather, there has been a celebratory atmosphere on The Mall, with periodic Mexican waves and police officers and security guards being cheered.
Among the spectators is Alexandra Hornyak, 57, from Montreal, Canada who will be watching from Green Park.
"I've known for years that I would want to attend this day, and the day that it was announced, I was driving to the office, and my husband just called me and said 'May 6'," she told the BBC.
"I knew exactly what it was. And I jumped on the phone to get a hotel reservation and we went from there."
Karen Daly, 54, from Birmingham, said, "We could have done this at home, but the atmosphere is so good and everyone's really happy."
"When the Queen died, we couldn't make it because we all couldn't get out of work. So we booked off as soon as we knew about the coronation."
Charles became King of the UK and 14 other realms in September, when his mother Elizabeth died after 70 years on the throne, the BBC reported.
Months of intense planning have gone into the coronation celebrations -- the 40th to take place at Westminster Abbey since 1066.
Unlike the procession route, there were no crowds outside the abbey, as it was closed to the public ahead of the service.
The service will be led by the Archbishop of Canterbury, Justin Welby, who will be assisted by the Archbishop of York, Stephen Cottrell.
"There have been wall-to-wall rehearsals this week, and indeed last week," he told BBC Radio 4's Today programme.
"None of us have done this for a very long time, so working out the choreography has been complex."
The high point of the ceremony will come when the St Edward's Crown is placed on the King's head, a moment that will be marked by the abbey bells and a gun salute in nearby Horse Guards Parade.
Camilla will be crowned alongside Charles -- and after the couple's long and often complicated relationship, she will now be officially described as "Queen Camilla".
The ceremony will emphasise diversity and inclusion, with more multi-faith elements than any previous coronation, with contributions from Jewish, Muslim, Buddhist and Sikh representatives.
A Bible lesson will be read by Prime Minister Rishi Sunak, who is a Hindu, and music will be sung in Welsh and Scottish and Irish Gaelic.
There will be women bishops taking part in the service for the first time in a coronation service that goes back almost a thousand years.
After the service, at around 13:00 BST, King Charles and Queen Camilla will travel in the Gold State Coach back to Buckingham Palace, in a spectacular mile-long (1.6km) procession, with 4,000 soldiers and 19 military bands.
When they reach the palace, it remains uncertain who will be seen with the King and Queen for the traditional balcony appearance.
There are plans for a flypast when the senior royals are on the palace balcony, but there will be concerns about the weather, with a forecast of cloud and showers.