The striking blond Wittstock, a 33-year-old former Olympic swimmer, will be transformed from commoner into princess in a civil ceremony in the throne room where Hollywood beauty Kelly married Albert's father, Prince Rainier III, 55 years ago.
The wedding continues Saturday, with an even more elaborate religious ceremony and opulent gala with a guest list including heads of state, European royalty and the creme de la creme of the fashion and sporting worlds.
With her bright blue eyes and even features, Wittstock has won comparisons with Kelly, who died in a car accident nearly three decades ago. Wittstock is expected to wear an ensemble by coveted Paris label Chanel.
Though Albert was long seen as among Europe's most eligible bachelors, the bespectacled 53-year-old prince long resisted marriage, and many in Monaco - known the world over for its lax tax laws and glamorous casinos - feared he might never tie the knot.
Friday's wedding was to be the first for both Wittstock and Albert, although the soft-spoken prince has acknowledged fathering two children out of wedlock.
Rumors raged over the past few days that a third illegitimate child had recently surfaced and prompted Wittstock to try to call off the wedding and return to South Africa.
The palace dismissed the stories as "ugly rumors" born out of jealousy. Speaking earlier this week on Monaco-Info TV, a top aide to the prince said the couple were "affected" by the rumors but added they were concentrating on last-minute preparations.
The guest list for Saturday's festivities included the kings of Spain, Sweden, Lesotho and Belgium, the presidents of France, Iceland, Ireland, Lebanon, Malta, Germany and Hungary, France's richest man, celebrated opera singers, top models and race car divers.
The Catholic ceremony will also take place in the palace, and is to be followed by a gala dinner by three-time triple-starred Michelin chef Alain Ducasse. Besides the Champagne and the South African wines, everything served at the sumptuous buffet is being sourced in a 10-kilometer (6-mile) radius from Monaco, Ducasse told The Associated Press.
More than 1,000 journalists from around the world have been accredited for the festivities, which kicked off late Thursday with a free concert by the legendary Los Angeles band The Eagles.
Albert and Wittstock attended the show, he in a dark suit, she wearing a strapless black pantsuit - a rather rock-and-roll sartorial choice for someone poised to become princess in under 24 hours.
