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 last evening with a boat ride across the sparkling waters of Stockholm bay and a dinner on a small city centre island.
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.
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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.
The Swedish media revealed the prince's budding relationship with the former glamor 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.