Turkey's justice minister said that any opposition challenge to a referendum that expanded President Tayyip Erdogan's powers would be rejected by the constitutional court, and Europe's human rights court had no jurisdiction on the matter.

The main opposition CHP party said on Wednesday it was considering taking its appeal for the referendum to be annulled to Turkey's Constitutional Court or the European Court of Human Rights after the country's electoral authority rejected challenges by the CHP and two other parties.

"If the opposition takes the appeal to the Constitutional Court, the court has no other option than to reject it," Justice Minister Bekir Bozdag told television news channel A Haber.

"It can also apply to the ECHR, but it cannot achieve a result there either, because the agreements Turkey signed do not give parties the right to apply."

Bozdag also reiterated government criticism of a report by European election observers who said the referendum, carried out under emergency law, took place on an "unlevel playing field".

The observers said a last-minute decision by election authorities to allow unstamped ballots to be counted "undermined an important safeguard and contradicted the law which explicitly states that such ballots should be considered invalid".

Bozdag said the report lacked fairness and objectivity. "Those who prepared this report are partial," he said.

Sunday's referendum narrowly backed the largest overhaul of Turkey's political system since the founding of the republic nearly a century ago, giving Erdogan sweeping authority over the NATO member-state.

But the tight result of a highly charged campaign laid bare divisions and triggered challenges from the opposition over its legitimacy.