Some analysts, especially in the US, are worried that the ECB's move amounts to printing money, which the central bank promised not to do.
"Let's face it, the ECB and the euro zone have just printed a lot of money, we've been doing it at this end," Arthur Hogan, global equity product strategist at Jefferies in the US, said.
But analysts in Europe disagree, saying that inflationary pressures are under control as long as the ECB will be taking money out of the markets via deposit auctions or other measures.
"I don't think they're printing money, because they said they'll sterilize it," Martin van Vliet, euro economist with ING Bank, told CNBC.com. "The effect on total liquidity will be neutral."
The goal of the euro zone bond buying is different from that of the Federal Reserve or the Bank of England, some analysts argue, because instead of stimulating the economy the ECB only wants to scare off speculators.
"The risk to speculate against the government is high now because you have an ECB quarterback in the background," Stefan Schneider, euro zone analyst at Deutsche Bank, told CNBC.com.
Confidence in Euro Hurt
But the ECB will have to keep buying government bonds for a long time before investors' confidence is restored, he added.
"I think this procedure might be in place for several years," van Vliet said. "But that doesn't mean they'll be active every day. They have this bazooka in their pocket."
The central bank did not give details of how much it is prepared to spend, what maturities it will target and from what countries.
Since Monday, central banks in the euro zone have been buying Portuguese, Greek and Irish debt across all maturities up to 10-year bonds for amounts of 25 million or 50 million euros at a time ($32 million to $64 million), according to dealers, but specific amounts or types of debt bought are not available officially.