"We are cooperating with the authorities and do not consider there to have been any wrongdoing by Citi or its employees," the bank said in a statement.
The Greek finance ministry on Wednesday said it had requested a judicial investigation into an email from "an international investment bank" as the resulting speculation over a debt restructure had "influenced" markets.
Greek police added that they had traced the electronic message to computer terminals at a "major global bank's subsidiary" in London from where it was sent to a large number of users, including Greek stock exchange addresses.
UK police had been asked for assistance via Europol, they said.
The Athens stock exchange shed 2.62 per cent Wednesday with Greek banks dropping 4.58 per cent but the general index on Thursday recovered with 2.26-per cent gains.
The email attributed to Citi, reprinted by Greek media, cited "increased noise over Greek debt restructuring as early as this Easter weekend."
"An email from brokers at an international investment bank referring to a restructuring of Greece's debt during the Easter weekend has been forwarded to the Athens prosecutor's office," the finance ministry said on Wednesday.
"Such rumours clearly lack seriousness and are ridiculous," it said.
"But because such dissemination of false news can create anxiety among the wider public, the finance ministry will henceforth request the identification and exemplary punishment of those responsible, exhausting all legal means," the ministry added.
A similar uproar occurred last month when another email posted on blog sites, reportedly traced to a middle-aged lathe turner on the island of Crete, said the country would go bankrupt on March 25, Greece's independence day.
Greece has repeatedly rejected talk that it is about to seek easier repayment terms from its creditors on a soaring 340-billion-euro ($491-billion) debt that many investors see as unsustainable in the near term.
The rate of return on Greek benchmark 10-year government bonds this week exceeded 14 per cent for the first time since the eurozone was created, reflecting fears that Athens will be unable to stabilise its public finances.
The finance minister insisted Wednesday that the debt is "viable" and that Athens still intends to raise money on the markets early next year despite currently exorbitant rates.
"The Greek debt is viable and its viability depends on how the (reforms) programme is run," Finance Minister George Papaconstantinou told reporters.
"Our aim is still to access markets in early 2012 to borrow at logical rates," he added.
Greece has already secured a repayment extension on its 110-billion-euro ($159-billion) bailout loan secured from the EU and the International Monetary Fund last year.
Several ruling party lawmakers, however, have called on the government to consider restructuring the nation's debt, as has Costas Simitis, the former prime minister who oversaw Greece's transition to the euro a decade ago.