U.S. stock index futures sank on Tuesday on continued worries over Greece's financial state and after a broad rally in the previous session that took the S&P 500 to its biggest day in two months.

Striking public workers challenged the Greek government's bailout-for-austerity deal with the European Union and the International Monetary Fund as investors worried about Athens' ability to enforce deeper spending cuts.

The plan to rescue Greece is a short term relief, and that's why we saw a rally yesterday. But just the fact that they had to make it to this point just reminds investors that we are not in a good situation, said Peter Boockvar, equity strategist at Miller Tabak & Co in New York.

S&P 500 futures were down 9.3 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 69 points, while Nasdaq 100 futures dipped 15.75 points.

Futures extended losses after a security alert was issued for a London subway station, but London Police later reopened the Aldgate East station on the edge of the financial district.

Drugmakers will be in view after both Merck & Co Inc and Pfizer Inc reported better-than-expected first-quarter profits.

Pfizer shares were up 1.9 percent at $17.24, and Merck shares were up 0.2 percent at $35.34 in premarket trade.

U.S. March factory orders, to be released by the Commerce Department at 10 a.m. <1400 GMT>, top the list of economic data. Economists in a Reuters survey expect a month-over-month drop of 0.1 percent, compared with an 0.6 percent rise in February. Also at 10, the National Association of Realtors will release figures for March pending home sales. Economists expect a rise of 4.0 percent rise month over month, versus an increase of 8.2 percent in February.

The financial sector will be in view as the U.S. Senate is set to cast its first votes on a sweeping Wall Street reform bill, with passage of a handful of uncontroversial amendments expected and a key procedural question still unsettled.

Energy shares may be in play as the massive oil spill in the Gulf of Mexico worsens, and BP Plc said it made progress toward capping the underwater well that ruptured almost two weeks ago.

Silver Lake and Warburg Pincus will buy Interactive Data Corp , the financial data provider, for $3.4 billion in cash.

(Reporting by Angela Moon; Editing by Padraic Cassidy)