"We've seen central banks buying at the short-end of the curve. They acted a little bit more aggressively than usual with bids worth 20-30 million euros," one trader said.

A second trader confirmed size and target of the purchases.

Late on Friday, Standard and Poor's downgraded the ratings of Italy, Spain, Portugal and Cyprus by two notches and the standings of France, Austria, Malta, Slovakia and Slovenia by one notch each.