A blackout on the publication of surveys was imposed on Nov. 18, but private polls are still being carried out and bookmakers say the "No" camp remains the clear favorite to win.
Economy Minister Pier Carlo Padoan sought to calm nervous markets, saying in a newspaper interview that there was "no risk of a financial earthquake" if "No" wins, though there may be "48 hours of turbulence".
European shares fell to a three-week low on Friday as investors stayed cautious ahead of Italy's vote, with Milan's blue chip index off 0.75 percent.
The bond market was more sanguine, with the gap between Italian and German 10-year bond yields, a key bellwether of investor sentiment, dipping to 165 basis points against a 2-1/2 year high of 190 points registered last week.
Market jitters have concentrated on Italy's banks, saddled with 360 billion euros ($380 billion) of bad loans, and most specifically on Monte Dei Paschi di Siena, its oldest and third largest lender.
The bank needs to raise 5 billion euros ($5.3 billion) by the end of the year to plug a capital shortfall or risk being wound down. Government officials say potential investors may be deterred by political instability if 'No' wins on Sunday.
With bookmakers odds suggesting a roughly 75 percent chance of a win for "No", speculation is rife on what Renzi will do in the event of defeat.
Renzi has said he will play no role in any unelected, so-called "technical" government, which Mattarella may try to put in place. Some of his allies have urged him to stay in power regardless of the result.