It bought a minimum of €10bn bonds in the second week of a programme supporting a rescue package masterminded by the International Monetary Fund (IMF).
The purchases typically take 2-3 days to settle, so the full amount bought in could be even higher than the total announced.
The rescue package is designed to buoy markets left deflated by the Greek sovereign debt crisis, uncertainty surrounding Germany’s decision to ban naked short selling and fears over tough new banking curbs being discussed in Washington.
So far it has had little effect on spooked Eurozone markets.
The move represented a screeching U-turn by the ECB, which had expressed reluctance to buy government bonds to stimulate markets.
It has not revealed the specific bonds it bought as part of the rescue package and has not placed a cap on the scheme. The ECB said it would offer banks the chance to deposit €26.5bn in one-week deposits to mop up extra liquidity created by the purchases.