Prince Harry and Meghan Markle are due to marry on Saturday afternoon and hundreds of people have travelled from all over the world to see the pair say "I do".

With Union Flag camping chairs and Royal bunting in hand, the overnight revellers began filling up the pavements of Windsor as early as 10am (7pm AEST) on Friday in a bid to secure the best vantage points.

Royal enthusiasts wrapped up warm to see out the night in the hope of catching a glimpse of the happy couple during the wedding of the year.

Nicky and Matt Pruner, from California, extended their holiday in the UK for an extra week so they could be in Windsor for the royal wedding.

Nicky Pruner, 62, said: "I camped out all night at Kate and William's wedding and I said I would never do it again - and here I am."

Matt Pruner, 63, said: "It feels like a festival, except this one has a meaning.

"It just gives hope to the world. We are very excited to be here."

Travelling a more modest distance to set up an overnight camp in Windsor was Claire Aston, 74, from Ealing, west London.

She said: "I slept out three nights for William and Kate's wedding and three nights for Charles and Diana's.

"You would think I'm old enough to know better than to camp outside but rather stay at home in a comfy bed. But I'm a Londoner - we can do this sort of thing!"

Ms Aston, who managed to catch a glimpse of William and Harry earlier in the evening as they greeted crowds outside the Castle, said she was looking forward to the big day.

"It's always a lovely crowd. You meet like-minded people with tales to tell," she said.

"I'm very excited for tomorrow. I wish them the very best of luck."

Australian Associated Press