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

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.

Fears over which nation might be next weighed on markets unconvinced that the aid package 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 International Monetary Fund warned of financial contagion unless the crisis was stopped in Greece.

The problem in the timing of quelling the debt concerns is that there won't be some event that calms people down. It will be a process as deleveraging takes time, said Peter Boockvar, equity strategist at Miller Tabak + Co in New York.

With global growth likely being tested over the next few months, U.S. economic data over the next three days will be a good measure of where we stand in the face of upcoming headwinds with ADP and ISM services specifically out today.

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 4.6 points and below 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 22 points, while Nasdaq 100 futures dipped 7.75 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. The stock rose 2.9 percent to $33.60 before the bell.

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.

Global 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.

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)