U.S. stocks fell for a second day on Wednesday as Greece's sovereign debt crisis spread and Moody's Investors Service warned Portugal could be next to have its debt downgraded.

Doubts about Europe's plan to rescue Greece and fears the debt problems could hinder global growth led buyers to safe-haven investments, boosting U.S. Treasuries and knocking stocks and the euro.

Data on the U.S. private sector job market and the economy's services sector were generally positive and helped stem stock losses.

Markets are waiting to see clear evidence the Greek fiscal consolidation is working and there are no implementation problems with the support package, said Zach Pandl, economist at Nomura Securities International in New York.

The Institute for Supply Management said the pace of growth in the U.S. services sector was unchanged in April compared to March. Earlier in the day, a separate report showed U.S. private labor sector added 32,000 jobs in April.

The Dow Jones industrial average <.DJI> was down 13.61 points, or 0.12 percent, at 10,913.16. The Standard & Poor's 500 Index <.SPX> was down 2.82 points, or 0.24 percent, at 1,170.78. The Nasdaq Composite Index <.IXIC> was down 11.72 points, or 0.48 percent, at 2,412.53.

Moody's is more likely to downgrade Portugal's credit rating after putting it on a three-month review than when it first put the country on negative outlook last year, a senior Moody's analyst said on Wednesday.

As investors fled from risky assets, the U.S. dollar <.DXY>, considered safe-haven investment, gained nearly 1 percent against a basket of major currencies. The euro fell below $1.29 for the first time in more than a year.

Oil prices fell below $80 a barrel for the first time since March, pressuring energy stocks. The S&P Energy Index <.GSPE> was down 1.2 percent and Chevron Corp shares lost 1 percent at $79.95.

Materials and industrial stocks have been under pressure this week as they are sensitive to the outlook of global economic growth.

The S&P Materials index <.GSPM> fell 0.2 percent. Dow Chemical Co was down 1.6 percent at $28.81.

The Chicago Board Options Exchange Volatility Index <.VIX>, Wall Street's favorite measure of investor anxiety rose 2.4 percent to 24.41 after closing at its highest level in almost three months.

Greek protesters clashed with police as tens of thousands of strikers marched against austerity plans in a crucial test of the government's resolve in enacting deep budget cuts in return for a massive bailout.

(Additional reporting; Editing by Padraic Cassidy)