Debt restructuring in Greece, Portugal and Ireland with write-downs for private creditors of 75 percent to 80 percent are needed to help stop Europe's debt crisis turning into a global meltdown, said BlackRock, one of the world's largest asset managers.

"Governments are falling, bond yields are zig-zagging by whole percentage points and markets around the world are locking up: the euro zone turmoil risks turning into a global crisis," BlackRock said in a research note on Monday.