"The assertions are completely made up out of thin air and factually wrong. They are either based on ignorance of the facts or are politically motivated comments (that) neglect the facts," Torsten Hinrichs told Austrian radio.
Critics include European Central Bank Governing Council member Ewald Nowotny, who said this week ratings agencies appeared to be applying a double standard in Greece's case.
Asked why S&P had not waited for the final details of a French plan to let investors roll over Greek debt before warning it could trigger a default, Hinrichs said:
"We did not reject the plan per se, but rather commented on how Standard & Poor's would react from today's perspective and from the information that is currently available. It is a purely hypothetical comment because as you correctly say the final plan has not yet been adopted."
The key issue was viewing how investors would be treated in comparison to bonds' original terms for repaying principal and interest, he said.
"It is very clear here that a significant extension of maturities and a delay in repayment are to be expected and that is not in line with the original promise of timely payment of interest and principal and to this extent such a plan would be a default."