Hundreds of Swedes gathered in the hot summer sun on Saturday to watch former reality show starlet Sofia Hellqvist become a real-life princess when she marries Prince Carl Philip in a lavish Stockholm wedding.

Royal watchers clutching blue and yellow national flags pitched rugs and deck chairs along the route that the newlyweds will take after exchanging vows and before returning to the palace for a 21-gun salute and wedding banquet.

About 550 guests will attend the ceremony including Britain's Prince Edward, Earl of Wessex and Princess Takamado of Japan.

The celebrations marking Sweden's latest royal wedding began Friday evening with a boat ride across the sparkling waters of Stockholm bay and a dinner on a small city centre island.

Hellqvist, a slender and elegant brunette, is a 30-year-old commoner known for her infectious gap-toothed smile and easygoing ways.

Her fiance, 36-year-old Carl Philip, is the second child of King Carl XVI Gustaf and Queen Silvia and holds the title of Duke of Varmland.

He is third in line to the throne, behind his elder sister Crown Princess Victoria -- who also married a commoner -- and her three-year-old daughter Princess Estelle. The prince will keep his title thanks to a change of rules in 1980 which also makes Hellqvist a princess through marriage.

"It is the first time a woman of the people marries a prince who is also marrying a princess," royal historian Goran Alm told public broadcaster SR.

A glamourous and openly affectionate couple equally known for their sporty lifestyle, Carl Philip and Sofia are keen to project an image of themselves as modern and open royals, and are often seen running errands in town or picking up pizza to go in Stockholm's chic neighbourhoods.

The Swedish media revealed the prince's budding relationship with the former glamour model in 2010, titillated by her racy past.

Hellqvist took part in 2005 in the Swedish reality show "Paradise Hotel", where young scantily-clad men and women stay at a luxury resort and compete to be the last contestant voted off the show amid intrigue, scheming and drama.

The year before that, at age 20, she had posed topless with a boa constrictor for the men's magazine Slitz, winning its Miss Slitz title.

'I don't regret anything'

The Swedish media was quick to publish the photos when the couple's engagement was announced in June 2014.

While the pictures may have raised a few eyebrows, Hellqvist's colourful past hasn't led to any major controversy.

"It's a shame there's still so much written about that," she told Swedish Television in a documentary about the royal family in January, referring to the racy photos.

"It was 10 years ago, I've gotten on with my life since then...I don't regret anything," she said.

Raised in a middle-class family in the small central Swedish town of Alvdalen, Sofia left Sweden for New York in 2005 where she studied accounting and worked as a yoga teacher and waitress before returning to Sweden.

In 2010, she and a friend founded the "Project Playground" charity to help disadvantaged children in South Africa, after having worked as a volunteer in several African countries.

She left her position with the organisation in March to focus on her new duties as princess following the wedding.

Contrary to her brother-in-law Chris O'Neill -- an Anglo-American businessman married to Carl Philip's sister Princess Madeleine and who often snubs royal events -- Sofia is popular among Swedes who admire her candour and commitment.