Thousands of people turned central London red, white and blue on Saturday, with a sea of Union Jack flags lining the streets for the coronation of King Charles III.

While the coronation service itself promises to be a sombre and sober affair, a celebratory atmosphere was building in the British capital and beyond, despite wet weather.

On The Mall outside Buckingham Palace, the most fervent royal fanatics got the party started hours before the ceremony began, with some camping out for days to secure a prime spot for the historic event.

As the royal procession passed on the way to Westminster Abbey, a forest of arms rose as people captured images of the king with their mobile phones.

Street vendors joined the hardcore supporters in the clamour to see the monarch's golden coach, with one man bellowing out "God Save the King".

"We've been here since five", said proud fan Alison Marschall, as she showed off her video of the procession.

But not everyone was in a celebratory mood, with scores of republican protesters -- many dressed in yellow and waving banners saying "not my king" -- gathered at Trafalgar Square and jeered.

- 'Good vibe' -

On Whitehall, crowds lined the street leading to Westminster Abbey, cheering marching bands as they passed.

The Union Jack was omnipresent, being waved on flags, emblazoned on T-shirts and painted on people's faces.

Among the early birds was Caryl Hall, 55, and her teenage children.

"I'm excited. There's a good vibe, good atmosphere -- friendly, happy, patriotic," said Hall, draped in a flag with a plastic crown on her head.

"It was hard to wake up the teenagers but this is part of history, we don't know if we'll see another one," added the South African, who was standing on a stool to get a good view.

Dave Giddings travelled from Scotland with his wife and son.

"It's being part of the future. It's an important thing," said the 41-year-old, sporting a gold crown and Union Jack sunglasses.

Torrential downpours on Friday failed to dampen spirits, with more bad weather forecast throughout Saturday.

Hillary, 72, and her daughter Jo, 47, caught an early train to join the crowds.

"It's living history. He is never going to be the queen (Elizabeth II) but he is our king and today we just want to celebrate," said Hillary, who only gave her first name.

The pair were loaded up with Union Jack hats, flags and picnic chairs, but in true British fashion were worried about the weather forecast.

- Celebration -

Many have flown in from abroad, including Christine Wilen, a retired nurse from Niagara Falls in Canada.

"I made the trip for the coronation," the 55-year-old, kitted out in the colours of her native Canada, told AFP.

"I'm very excited to be here, to be part of this history. I've always been a monarchist," she added.

Recent polls indicate waning support for the royals, especially among younger people, and at Trafalgar Square, republicans held aloft their protest placards as the procession passed.

Hours earlier, London's Metropolitan police arrested several organisers from the anti-monarchy group Republic, in a move denounced by Human Rights Watch as draconian and "alarming".

The demonstrators were far outnumbered by monarchists, including Caba Mendes, 21, from London.

"It's a great day for the country, I can't wait," he said while holding his phone with a selfie stick for the procession on The Mall.

Helen Rimmer, who travelled from Cumbria, northwest England, on Friday night, secured her spot on Whitehall at 4:45 am.

"It's a very special occasion, especially for our country and the Commonwealth. It's the atmosphere, just everything about it really. It's just a big celebration. It's great."