Stocks were set to rise sharply at the open on Thursday after the European Central Bank surprised markets by cutting interest rates and as a Greek referendum on its euro zone membership was put into question.

European shares extended gains to rise nearly 2 percent after the ECB move. The region's bank sector, a key focus because of its sovereign debt exposure, rose 3.6 percent and a Greek bank index rose 13.2 percent.

The move is more symbolic than anything as moving rates from 1.5 percent to 1.25 percent isn't going to move the needle for a region that is suffering from structural economic stagnation and too much debt, said Peter Boockvar, equity strategist at Miller Tabak & Co in New York.

Psychologically though, markets love it just as it loves every easing move by the Fed. Cheap money, party on.

The European Central Bank cut its main interest rate by 25 basis points to 1.25 percent as the euro zone's worsening debt crisis outweighed the concern over persistently high inflation.

S&P 500 futures rose 13.6 points and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures gained 125 points, and Nasdaq 100 futures rose 16.25 points.

Greece's government was on the brink of collapse, casting doubt on plans for a referendum on staying in the euro, as European leaders talked for the first time of a possible Greek exit to preserve the single currency.

European leaders agreed last week on a plan to reduce Greece's debt and strengthen the region's bailout fund in a move that helped spark a month-end rally. October was the best month for U.S. stocks in 20 years.

(Reporting by Rodrigo Campos; additional reporting by Angela Moon)