By Tommy Stubbington And Dan Strumpf 

Investors are steeling themselves for a rough ride Monday after Greek voters appeared to drive a deeper wedge between the country and its creditors, a move that piles pressure on stocks, bonds of some European countries and the euro.

The common currency tumbled in early Asian trading on Monday after Greece was projected to have voted "no" to creditors' austerity demands in Sunday's referendum. U.S. stock futures also fell sharply late Sunday following the Greek vote.

"It's going to be very messy in the markets," said Jeanne Asseraf-Bitton, head of cross-asset research at Lyxor Asset Management. "It doesn't even look like a close call. That will come as a surprise to many."

Prime Minister Alexis Tsipras said a no vote would force Greece's creditors to compromise. But the creditors have said a rejection of the bailout terms would severely hurt the chances of a deal, putting the country at risk of an exit from the currency bloc--a prospect that has rattled markets as talks intensified over the past few weeks.

"The majority of our clients went into the weekend expecting a 'yes' result," Francesco Garzarelli, co-head of markets research at Goldman Sachs Group Inc., wrote in a note to clients shortly before polls closed Sunday.

He expects a definitive no outcome to increase the likelihood of Greece's exit from the eurozone.

Investors expect a bout of market tumult in the U.S. and Europe following Sunday's vote.

Eurozone equities have drifted lower over the past month after a strong start to the year, with the Euro Stoxx 50 losing nearly 4%. Investors are bracing for the most volatile markets since the height of the region's debt crisis in 2012. The VSTOXX index--a eurozone measure of how much investors are paying for options to protect their equity portfolios against price swings--climbed to its highest level in three years on Friday.

The events in Greece add another wrinkle to U.S. stocks, which have all but flatlined this year against the backdrop of an uneven economy and slowing profit growth. In the U.S., futures on the S&P 500 index fell 30 points, or 1.5%, to 2040 shortly after electronic trading began at 6 p.m. ET Sunday. E-mini Dow Jones Industrial Average futures shed 252 points, or 1.4%, to 17397. E-mini Nasdaq-100 futures lost 65 points, or 1.5%, to 4364.

"It will be a negative for markets across the board," said Brian Burrell, equity research analyst at Thornburg Investment Management, which manages $61 billion.

"I wouldn't be surprised if we had weakness [Monday] and we had a buyer's strike," said Michael O'Rourke, chief market strategist at broker JonesTrading. "There's so many uncertainties out there. Most people are going to move to the sidelines and see how it shakes out."

Markets have swung sharply in recent weeks on the dramatic twists and turns in the negotiations. On June 22, Greek stocks climbed 9%, while Germany's DAX index surged 3.8%, its biggest one-day gain since August 2012, after a deal appeared to be in sight. The following Monday, stocks around the world fell sharply after a weekend breakdown in talks.

Greece's stock market was on lockdown last week and won't reopen until Tuesday at the earliest, along with the country's banks. Trading in Greek government bonds also has been halted on some trading platforms after they tumbled last Monday.

Most investors expect bonds issued by other highly indebted nations like Italy, Spain and Portugal to weaken, but that the spillover from Greece will be less pronounced than at the height of the eurozone crisis in 2011 and 2012--even after Sunday's vote. Those countries' bonds have in the past been vulnerable to fears that Greek departure from the eurozone could foreshadow a wider splintering of the currency bloc.

"Obviously, the world is a little more stable than it was in 2010 and 2011, but the risks are real and legitimate in that the bonds are far more expensive today," said Mr. O'Rourke.

Unlike a few years ago, few Greek government bonds remain in private hands, limiting the potential for a default to cascade through markets. The European Central Bank's bond-buying program is seen as a backstop to bond markets because the bank could scale up purchases to protect other vulnerable eurozone members.

"The key for the road ahead is that the ECB delivers protection from contagion," said Mads Pedersen, head of discretionary asset allocation at UBS Wealth Management, which oversees around $2 trillion of assets.

Beyond the short-term volatility, many investors are confident in the resilience of eurozone assets, betting that the fallout from Greece won't derail an economic recovery in the region.

Some believe the Greek crisis could bolster the currency bloc in the long run.

"'If it doesn't kill you, it makes you stronger' is the old saying. In a way, that's appropriate here," said Brian Jacobsen, chief portfolio strategist at Wells Fargo Funds Management.

Write to Tommy Stubbington at tommy.stubbington@wsj.com and Chiara Albanese at chiara.albanese@wsj.com