Markets are now turning their attention to President Jean-Claude Trichet's news conference at 1230 GMT, when he will explain the decision and may fill in some of the missing details of the bank's plan to buy 60 billion euros' ($84 billion) worth of covered bonds.

The ECB also kept its overnight deposit rate, which is acting as a floor for money markets, at 0.25 percent, and left its marginal lending rate at 1.75 percent.

All but one of the 82 economists polled by Reuters had expected the ECB to leave rates on hold. One said it would cut them by 0.25 percentage points.

The ECB has lowered its interest rates from 4.25 percent since October after the financial crisis intensified and inflation risks weakened.

Euro zone consumer prices fell for the first time in June and the economy is shrinking fast, although some data has shown signs of improvement.

Markets are also keen to see whether Trichet gives any signals that the ECB plans to keep rates low for a lengthy period, or alternatively, whether they could be reduced further.