Photos have surfaced of guests, including Justin's Saturday Night Live partner in crime Andy Samberg, sipping champagne at an open air gathering in Southern Italy.
Meanwhile sources have told American publication Us Weekly that proceedings kicked off on Tuesday, with an impressive fireworks display taking place the following evening.
Red, green and white blasts illuminated the sky in a little seaside town and a large van and several spotlights were spotted in the area.
But there has been no confirmation as to whether the display was in fact part of the wedding celebrations or completely unrelated.
An insider told the magazine Jessica and Justin are taking "extra precautions" to keep details under wraps.
The SexyBack star's grandmother let slip the couple will marry in Italy.
Sandy Bomar told Star magazine that the she and her husband Bill would miss the nuptials as Mr Bomar's health problems make it too difficult to travel.
"Unfortunately, we won't be able to go to Italy," she told the publication.
Justin recently bid farewell to single life in the traditional manner by taking a group of 25 of his closest friends off on a stag do.
The boys were spotted partying on Las Vegas' famous strip before they decamped to Mexico.
In Cabo, the singer-turned-actor and his crew played beach volleyball and sampled the nightlife.