Greek billionaire Athina Roussel Onassis is to wed a Brazilian equestrian star Saturday, in what is expected to be an extraordinarily elaborate ceremony.

The 20-year-old heiress will marry 32-year-old Alvaro Affonso de Miranda on the grounds of an art museum in Sao Paulo's upscale Morumbi district. Reports say she will wear a dress by designer Valentino, and 1,000 bottles of fine champagne will be served to as many as 750 guests.

The couple have asked guests to donate money to charities in lieu of gifts.

The bride is the granddaughter of the late Greek shipping tycoon Aristotle Onassis. Media reports say she inherited an empire worth between $1-3 billion on her 18th birthday.

The couple met in 2002 in Belgium, and share a passion for horses. They live in Sao Paulo.

Some information for this report provided by AP and Reuters.