Meanwhile, German Foreign Minister Sigmar Gabriel said that despite a landmark referendum in Turkey that will grant sweeping new powers to the nation's president, Turkey should not leave North Atlantic Treaty Organisation.
In a combative speech at the presidential palace in Ankara on Monday evening, Erdogan dismissed the protesters as well as the opposition party complaints, saying the "debate about this issue is now over".
Allegations of vote rigging are uncommon in Turkey, Soner Cagaptay of the Washington Institute for Near East Policy tells NPR.
The "Yes" camp won Sunday's referendum with 51.41 percent of the vote but the aftermath has been shadowed by opposition claims of blatant vote-rigging and angry protests in parts of Istanbul.
As foreign leaders take an increasingly hard line in the face of mounting evidence of election fraud, and government supporters consolidate on their win, the chasm between Turkey and the west will continue to widen, says the BBC. The US state department noted both issues in a more cautious, less laudatory statement issued a few hours earlier.
In Istanbul, hundreds of "no" supporters demonstrated in the streets on Monday night, chanting "thief, murderer, Erdogan" and banging pots and pans.
Gunal also voiced his support for Erdogan's announcement that he will study the possibility of restoring the death penalty in Turkey.
"These violations contravene OSCE commitments, Council of Europe standards and other worldwide obligations regarding freedom and equality in the campaign", it said, referring to the Organization for Security and Cooperation in Europe, an intergovernmental body that regularly reviews human rights practices in Europe and beyond.
The ministry said the statements made by OSCE officials before the referendum showed they "arrived in Turkey with prejudices and ... disregarded the principles of objectivity and impartiality".