Large-scale purchases of short-term government bonds would drive up their price and push down their interest rate, or yield, taking some pressure off of financially stressed governments such as Spain and Italy.
"We will have a fully effective backstop to avoid destructive scenarios," ECB President Mario Draghi said at a press conference, in which he also defended the euro currency union as "irreversible."
After the ECB plan was announced, the yields on government bonds across Europe fell and stock markets rallied.
"This is a potential game-changer," says Jacob Kirkegaard, research fellow at the Peterson Institute for International Economics. "This is the first time the ECB has committed its balance sheet in this way. And the way it is done is politically sustainable in Europe."
The ECB's 23-member governing council approved the plan with only one dissent. The head of Germany's Bundesbank, Jens Weidmann, opposes the plan, arguing that the ECB is moving too far in the direction of financing government deficits, which is prohibited by the European Union treaty.
The ECB's pledge of support came with an important caveat: Countries that want the central bank to help with their borrowing costs must first ask the 17 countries that use the euro to buy their bonds with existing bailout funds and they must submit their economic policies to the scrutiny of the International Monetary Fund.
That puts immense pressure on financially stressed countries such as Spain and Italy — which have been reluctant to seek help from their euro partners — to take the next step. Both countries face borrowing rates that are in the upper range of what's sustainable over the long-term.
Spanish Prime Minister Mariano Rajoy refused Thursday to make any commitment to trigger the ECB bond-buying. "When I have something new, I'll tell you," he told reporters at a press conference that was held after he met with German Chancellor Angela Merkel.
Italian Prime Minister Mario Monti praised the plan as an "important step forward" but said any decisions on Italy's potential request for aid were "premature."
If Spain and Italy were to accept aid from the eurozone's bailout funds, it would put them in the same company as Greece, Ireland and Portugal — something the two countries have been wary of.
An earlier ECB effort to drive down rates by buying bonds was limited in size — the ECB spent €210 billion ($264 billion) — and did not have lasting impact. The ECB ended that program Thursday.
The ECB first said its new bond-buying plan was in the works on Aug. 2, and markets have climbed steadily since then. Investors across the globe greeted the release of a more detailed plan with further enthusiasm.
The Dow Jones industrial average climbed 244 points to 13,292. Germany's DAX index and France's CAC-40 index each rose roughly 3 per cent. The euro rose 0.33 per cent during the day to $1.2636.
The interest rates on the three-year and ten-year bonds of Spain and Italy fell in a sign of market anticipation of ECB bond-buying.
"If the current positive market mood continues, the ECB may end up having to buy nothing," said Marco Valli, an analyst at UniCredit bank in Milan.
Other analysts cautioned that while the ECB plan would provide short-term relief to European countries and financial markets, it doesn't address underlying economic weakness across the region, which could persist for years.
Put simply, buying government bonds does not stimulate growth, which has been hurt in Spain, Italy and other countries, in part, by the need to rein in government spending.
Six countries in the eurozone are in recession — Greece, Spain, Italy, Cyprus, Malta and Portugal — and unemployment across the region is 11.3 per cent.
The ECB revised lower its economic forecast for the 17-nation eurozone economy on Thursday, saying it would shrink between 0.2 per cent and 0.6 per cent in 2012. The bank left its benchmark refinancing rate unchanged at 0.75 per cent, a record low.
Here are some of the details of the ECB's bond-buying plan:
— To trigger ECB bond-buying, a country must first ask its euro partners to use one of two bailout funds to buy their bonds at auction. The ECB would then buy bonds with remaining maturities of one to three years on secondary markets until it was satisfied that their interest rate had fallen to a reasonable level.
— The ECB would reveal the amount of bonds it has purchased on a weekly basis. It would reveal their average maturity and the countries that issued them on a monthly basis.
— The ECB will have the same status as private creditors holding bonds in a country. This pledge is intended to reassure private bond investors, who would otherwise worry about being the first to take any losses.
— For each euro the ECB spends on bond-buying, the bank plans to withdraw an equivalent amount from the financial system, by taking deposits or selling notes. This is meant to address concerns that by pumping money into the financial system the ECB could incite inflation.