Turkey's minister for European Union affairs lashed out at what he called politically motivated comments against the national electoral board's decision to accept unstamped ballots in Sunday's referendum.

Speaking to reporters in the capital Ankara on Tuesday, Omer Celik defended what he said were Turkey's strong legal framework and transparent process for elections.

"Politically motivated comments against the YSK's decision to accept unstamped ballots are wrong," Celik said, referring to the electoral board by its Turkish acronym.

Turkey's main opposition began a battle on Tuesday to annul a referendum handing President Tayyip Erdogan sweeping new powers, while the bar association and an international monitor said an illegal move by electoral authorities may have swung the vote.

The bar association said a last-minute decision by the YSK to allow unstamped ballots in the referendum was clearly against the law, prevented proper records being kept, and may have impacted the results.