FRANKFURT — In what seemed a rare communications blunder by one of Europe’s most politically polished technocrats, the president of the European Central Bank, Mario Draghi, set off a market backlash Thursday by investors who were apparently hoping for a bold euro rescue plan but instead heard a general statement of good intentions.

Draghi signaled that the ECB, in what would be a marked departure from current policy, was willing to start buying government bonds to hold down the borrowing costs of troubled eurozone countries like Spain.

But the bank put the onus on political leaders to move first, and left open some crucial questions about how quickly and forcefully it would seek to tame unruly financial markets.

Coming almost a week after Draghi incited a market rally by pledging to ‘‘do whatever it takes to preserve the eurozone,’’ his admission Thursday that whatever it takes might take weeks or months resulted in a Draghi rout.

Stock indexes started falling quickly and deeply in Europe almost as soon as Draghi began speaking at a news conference. And the downdraft carried over to Wall Street after the US markets opened. Borrowing costs for Spain and Italy, as measured by the yields on their bonds, spiked back up almost to the levels they were at before the rally started a week earlier.

“From a communication point of view, he misguided the markets,’’ said Joerg Kraemer, chief economist at Commerzbank in Frankfurt. ‘‘He raised expectations which he could not fulfill.’’

Kraemer and other economists had warned in recent days that market expectations were too high following Draghi’s vow in London last week and his assurance that whatever the ECB might be hatching, ‘‘it will be enough.’’

The ECB is prepared to ‘‘undertake outright open-market operations of a size adequate to reach its objective,’’ Draghi said Thursday. Interest rates demanded by investors ‘‘that are related to fears of the reversibility of the euro are unacceptable,’’ Draghi said, ‘‘and they need to be addressed in a fundamental manner.’’

“It’s up to the politicians — the same old story we've heard the last two years,’’ said Carsten Brzeski, a senior economist for the Dutch bank ING.

Investors may also be facing the reality that ECB attempts to lower borrowing costs for Spain and Italy could be constrained by continuing German opposition to bond buying. On Thursday, Draghi acknowledged that Jens Weidmann, president of the German central bank, opposed the general statement on bond purchases that the other 22 members of the ECB’s governing council had agreed to earlier in the day.

At that meeting, the governing council decided not to lower the central bank’s benchmark interest rate. The ECB’s announcement of that news 45 minutes before the start of Draghi’s news conference had no effect on the financial markets. That rate is already at a historical low of 0.75 percent, and any further reduction would be more symbolic than meaningful.