Buble, in a gray suit, and the actress, wearing a lilac chiffon dress with silver high heels, posed for a mob of fans outside the civil registry. Lopilato, 23, tossed her bouquet of purple orchids into the crowd as some fans threw rice and red rose petals.
Lopilato posted messages to her more than 500,000 Twitter followers throughout the day, thanking her fans, family and the media. "How beautiful it all was!" she wrote.
The couple will say their 'I Do's' in two more ceremonies planned for next month: one with 300 guests at a mansion outside of Buenos Aires and another in Vancouver.
Buble, 35, won two Grammys in February for Best Traditional Pop Vocal Album for "Crazy Love" and Best Male Pop Vocal Performance for "Haven't Met You Yet," and is one of North America's top-grossing concert entertainers. Lopilato made her name as a model and actress in Argentine sitcoms and soap operas, including "Rebel Way," "Little Girls," "Married With Children" and "Pirate Soul."
The Canadian singer dated British actress Emily Blunt for several years before he met Lopilato in 2009 during a South American concert tour. Lopilato was romantically linked to tennis player Juan Monaco in 2008.
The couple has homes in Canada, Los Angeles and Buenos Aires province.
The Associated Press contributed to this report.