Europe's hopes of containing Greece's credit crisis dimmed as the country's debt woes spread to Portugal, sparking a selloff in markets across the globe and testing the European Union's ability to protect its common currency.

The euro tumbled to its lowest point in a year against the dollar after Standard & Poor's Ratings Services cut Portugal's credit rating two notches and downgraded Greece's debt to "junk" territory, a first for a euro-zone member. The move is bound to worsen Greece's already dire fiscal situation and hamper a recovery.

The news sent the bond yields in both countries soaring, a sign of distress.

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