Political parties in Greece have criticised IMF head Christine Lagarde for suggesting that Greeks were avoiding paying taxes.
Socialist leader Evangelos Venizelos accused Ms Lagarde of "insulting the Greek people".
Left-wing leader Alexis Tsipras insisted: "Greek workers pay their taxes, which are unbearable."
In her interview with the UK's Guardian newspaper, Ms Lagarde suggested it was payback time for Greece.
She said she was more concerned about poverty-stricken children in sub-Saharan Africa than Greeks hit by the economic crisis.
Greece has promised to implement tough austerity measures in return for a multi-billion euro EU-IMF bailout. But the deal is under threat following inconclusive elections in May.
After thousands of angry messages were posted on her Facebook page, the head of the International Monetary Fund wrote that she was "very sympathetic to the Greek people and the challenges they are facing".
"That's why the IMF is supporting Greece in its endeavour to overcome the current crisis," she said.
Mr Venizelos told an election rally that he welcomed Ms Lagarde's Facebook message, but added: "Nobody can humiliate the Greek people during the crisis.
"I say this today addressing specifically Ms Lagarde... who with her stance insulted the Greek people," he said.
Mr Tsipras, whose Syriza party is one of the two main contenders for the 17 June election, said: "The last thing we seek in Greece is her sympathy. Greek workers pay their taxes, which are unbearable.
"For tax-evaders, she should turn to Pasok andNew Democracy [previous coalition government partners] to explain to her why they haven't touched the big money and have been chasing the simple worker for two years."
French minister Najat Vallaud-Belkacem told France's Canal+ TV that Ms Lagarde should not have made the comments.
"I find (her point of view) rather simplistic and stereotypical. I think that these days it shouldn't be about trying to teach people a lesson," she said.
French far-left politician Jean-Luc Melenchon even said Ms Lagarde should resign over the comments.
"What gives her the right to speak in this manner to the Greeks?" he said in an interview with France 3 TV.
Fears are growing that Athens may be forced to leave the eurozone if the June election produces a government opposed to the bailout deal.
That could possibly trigger a run on banks - not only in Greece but in other eurozone nations, experts warn
In the interview published on Friday, Ms Lagarde said: "As far as Athens is concerned, I also think about all those people who are trying to escape tax all the time. All these people in Greece who are trying to escape tax.
"I think they should also help themselves collectively."
She added: "I think more of the little kids from a school in a little village in Niger who get teaching two hours a day, sharing one chair for three of them, and who are very keen to get an education. I have them in my mind all the time.
"Because I think they need even more help than the people in Athens."
When asked if she was saying to the Greeks that it was now payback time, Ms Lagarde responded: "That's right."