LONDON, March 21 (Reuters) - Distressed debt investors and large hedge funds are buying up Credit Suisse (CSGN.S) additional tier-1 bonds at rock-bottom prices after they were written down to zero in the Swiss bank's rescue by cross-town rival UBS (UBSG.S). AT1 bonds issued by other European banks tumbled on Monday as the treatment of Credit Suisse AT1 bondholders highlighted the risks of this type of debt. Buyers have included a mixture of hedge funds and deep distressed debt funds, which Southey expected would need to hold the bonds for an extended period before they paid off. Some of those buyers intend to join groups that would litigate to improve odds on cashing in on the bonds, Southey said. "It's quite possible that we will see demand from buyers of subordinated bank debt to have more explicit protections written into these bond prospectuses in the future."

---