Wall Street set to stumble on the Greek 'no' vote

U.S. stock markets were expected to open lower on Monday on heightened fears of a possible Greek exit from the euro zone after the country's voters rejected austerity measures demanded in return for much-needed cash.

Japan's blue-chip Nikkei closed down just over 2 percent, while European shares opened broadly lower as investors reacted to Sunday's overwhelming 'No' vote by Greek voters.

That set the tone for trade in U.S. stock futures, with Dow Jones industrial average futures down more than 100 points in early London trade – indicating a weak open for Wall Street shares which reopen after being closed Friday for the Independence Day holiday.

"After the comprehensive victory for the 'No' camp in yesterday's Greek referendum, eventual 'Grexit' now looks more likely than not," analysts at Daiwa Capital Economics said in a note. "The response of policymakers over the next 48 hours will be key."

"Unless a deal can be reached swiftly the Greek banks and the Greek government will simply run out of funds making the introduction of a parallel currency soon inevitable," they added.

Greece, which last week defaulted on a major loan to the International Monetary Fund and has imposed capital controls to limit an exit of cash, is in uncharted territory following Sunday's referendum.

The country's outspoken Finance Minister Yanis Varoufakis resigned on Monday, removing an obstacle to talks with Greece's creditors.

Read More'Toxic' Varoufakis is out: Time for a deal?

The European Central Bank meanwhile holds a conference call later in the day to decide how long to keep Greek banks afloat, while euro zone leaders are to hold an emergency summit on Greece on Tuesday.

The Institute for Supply Management's June service sector survey is due out at 10:00 a.m. ET and may take investors' focus briefly away from Greece to the U.S. economic outlook.

After last Thursday's softer-than-expected U.S. non-farm payrolls report, expectations for a September interest rate rise from the Federal Reserve have been scaled back.

Read MoreWhen markets get past Greece, they'll watch Yellen

Elsewhere, U.S. Treasury yields were broadly lower as jitters over Greece sent investors fleeing into safe-haven bonds. The yield on the benchmark 10-year note was down 8.6 basis points at 2.30 percent.

The dollar was about 0.4 percent firmer against the euro at 1.1072 and down 0.2 percent at 122.64 yen.

Earnings results due this session include A Schulman, Helen of Troy, Stoneridge and Walter Energy.