The swap, which was taken up by 83.5 percent of Greece's private creditors on Friday, was needed for the release of a €130 billion, second aid package from eurozone partners for Athens to help it avert a default.
"The Greek debt exchange offer is interesting on a financial level, even for small debt holders," an SdK statement released late Friday acknowledged. "But its technical application is totally unacceptable for small investors," it added.
"The Greek government's project ... to force Greek bond holders to take
part in this partial default via collective action clauses violates fundamental legal principles," the statement said.
SdK head: fatal signal to investors
It quoted SdK director Daniel Bauer as criticising a "fatal signal to all investors" that would make "Europe a banana republic."
SdK said it was studying legal means at its disposition, and foresaw a "judicial process that lasts several years and certainly ends before the European Court of Justice."
Greece has triggered collective action clauses to force holdouts to accept bond swap aimed at erasing more than €100 billion ($132 billion) in debt and unlock a new bailout, a government source said Friday.
The decision was taken after Greece agreed with its eurozone partners that the move was the best way to ensure maximum participation in the deal.
Private creditors are to take losses of more than 50 percent on their holdings of Greek sovereign debt under the swap.