U.S. stock index futures fell on Wednesday as investors continued to anxiously eye the Greek sovereign debt crisis and its potential contagion.

Tens of thousands of striking Greeks protested against austerity plans, testing the government's determination to carry out draconian budget cuts in exchange for billions of euros in aid.

Fears over which nation might be next weighed on markets unconvinced that the massive bailout for Greece would stop the crisis from spreading to other vulnerable euro zone countries like Spain and Portugal.

German Chancellor Angela Merkel and the head of the IMF warned of financial contagion unless the crisis was stopped in Greece.

World stocks fell to eight-week lows and the euro hit a one-year trough on Wednesday.

Wall Street had its worst sell-off in three months on Tuesday and the CBOE Volatility Index <.VIX>, Wall Street's so-called fear gauge, finished at its highest level in almost three months.

On the macro front, ADP releases its April employment report at 8:15 a.m. EDT. Economists in a Reuters survey expect 30,000 jobs were created in April versus a loss of 23,000 jobs in March. The report serves as a precursor to the all important non-farm payroll figures due on Friday.

At 10:00 a.m. EDT, the Institute of Supply Management releases its services sector index for April. Economists expect a reading of 56.0 versus 55.4 in the prior month.

S&P 500 futures were down 3.6 points and in line with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 27 points, while Nasdaq 100 futures dipped 8 points.

Time Warner Inc posted a better-than-expected quarterly profit early Wednesday after a strong rebound in advertising sales at its cable television networks and magazines as well as strong DVD sales.

Oil fell below $82 a barrel a day after suffering its steepest one-day percentage loss in three months, on rising oil inventories and a firm dollar.

A flotilla of nearly 200 boats tackled a massive oil slick in the Gulf of Mexico, taking advantage of calm weather to intensify containment efforts.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)