"We couldn't be happier to begin our new journey together surrounded by our closest family and friends," the couple tell PEOPLE.
The pair, who have been dating since May, got engaged just before Christmas.
"Getting married is the next step when you are totally in love and comfortable with someone," a source tells PEOPLE. "Cameron has found true happiness."
Over the weekend, Madden, 35, and Diaz, 42, hosted an intimate rehearsal dinner at their Beverly Hills pad for family and friends, including Drew Barrymore, Joel Madden and Nicole Richie, who introduced the two, a source tells PEOPLE.
The pair were also spotted at Il Cielo Saturday night, where "they were with a small group and had a really romantic table setting with rose petals and candles," says a restaurant onlooker. "It looked like they were celebrating."
Diaz also celebrated her impending nuptials with a bachelorette party over the weekend, says the source. The Annie star spent Sunday night at a Los Angeles-area hotel with her girlfriends.
Party rental trucks and bunches of white flowers began arriving Monday morning to the gated community where Diaz and Madden live. And celebrity wedding planners Yifat Oren and Stefanie Cove (who planned the couple's rehearsal dinner and has planned weddings for Reese Witherspoon, Anne Hathaway, Natalie Portman and Diaz's close friend Barrymore) were also spotted arriving at their home Monday.
A huge tent was put up on Diaz's property for the ceremony and reception – the latter dimly lit with lots of candles and mostly white flowers. Witherspoon and Gwyneth Paltrow were among several celebrity guests in attendance. Benji's twin brother was also spotted arriving at their home in the early afternoon.
"There is a lot of security and they are trying to keep the wedding hush-hush by telling neighbors that Cameron is having a pre-Golden Globes party tonight," another source tells PEOPLE. "It will start past dark. They are setting things up in the lush backyard."
The wedding was held in front of 90-plus guests – including bridesmaids Barrymore, Richie and Diaz's sister Chimene Cain, as well as ring bearer and nephew Sparrow Madden – took place inside the home while the reception was held under a tent.
"It was a big wedding, but they still managed to make it feel very personal. Only their families and very special friends attended," a source tells PEOPLE. "It all felt very down-to-earth and sweet. Guests arrived smiling and almost seemed giddy. Everyone was excited to be a part of the wedding."
Reporting by PERNILLA CEDENHEIM and MARIAH HAAS