Turkey's high electoral board (YSK) rejected appeals from the country's main opposition parties to annul the referendum results, the board said in a statement on Wednesday.

The main opposition Republican People's Party (CHP) and the pro-Kurdish HDP had called on the electoral board to annul Sunday's referendum because unstamped ballot papers were included in the count. They argued that this contravened Turkish electoral law. The board overwhelmingly voted to reject the parties' appeals.

"HDP, CHP and Vatan Party appeals regarding the April 16 referendum were discussed separately and as a result of evaluations, the appeals were rejected with 10 votes against and 1 vote in favor," the electoral board said. The board also published past rulings on the validity of unstamped ballots.

Mehmet Hadimi Yakupoglu, the CHP's representative at the board, said that they were prepared to take the decision to the constitutional court and even to the European Court of Human Rights if necessary.

Sunday's vote was narrowly won by the "yes" camp, granting sweeping powers to President Recep Tayyip Erdogan.

The Turkish electoral board came under fire for including ballot documents in envelopes without an official stamp

Post-referendum protests

Earlier on Wednesday, Turkish Prime Minister Binali Yildirim warned against growing street protests triggered by widespread allegations of voter fraud in Sunday's referendum.

Yildirim said the opposition parties have the right to file objections, but "calling people to the street is wrong and is outside the line of legitimacy," Yildirim said, telling CHP leader Kemal Kilicdaroglu to "to act more responsibly."

Kilicdaroglu fired back, saying that protesting was a civic right.

Thousands of protesters accusing the ruling AK Party of election fraud have taken to the streets of Istanbul, Ankara and other cities since Sunday's vote, with another wave of demonstrations planned for Wednesday evening. At least 38 people have been arrested, including several for planning protests, according to Turkish media reports.

Unofficial results have Erdogan winning the referendum by a slim margin, with 51.4 percent voting "yes." But the claim of victory has been overshadowed by polarization and questions of legitimacy on an issue that will bring monumental change by transforming the country from a parliamentary into a presidential system.

No level playing field?

The HDP, the third largest party in parliament, filed a complaint with the electoral board on Wednesday calling for the referendum to be annulled after the CHP filed a similar appeal on Tuesday. In the months before the referendum, more than a dozen HDP parliamentarians were thrown in prison on terrorism charges as were hundreds of party members.

"This referendum will forever remain controversial," HDP deputy chairman Mithat Sancar told reporters. "You cannot build a change in the political system on such a controversial and unfair referendum."

The opposition has complained about an unlevel playing field before the referendum and irregularities during the vote, claims echoed by election monitors from the Organization for Security and Cooperation in Europe (OSCE) and by the European Union.

Accusations of voting irregularities have been bolstered by unverified videos and pictures on social media purporting to show ballot box stuffing, "no" votes being thrown away, and election officials stamping ballots with "yes" votes. Many of the irregularities were reported in traditional Kurdish strongholds of the HDP long opposed to AK Party rule.

The main target of the legal complaints are directed at the high electoral board for ruling during the referendum that some 1.5 million unstamped ballots and envelopes would be counted as valid. According to Turkish election law, unstamped ballots and envelopes are invalid, in a bid to limit the scope for fraud.

Michael Link, the top election observer for the OSCE, told a group of local German newspapers on Wednesday that Turkish authorities were not cooperating in addressing alleged election manipulation.

He also criticized the YSK, saying, "the quickly-made decision of the electoral commission to validate incorrectly stamped or entirely unstamped ballots is a violation of Turkish law."

Turkish Foreign Minister Mevlet Cavusoglu on Wednesday accused the OSCE of bias and deliberately putting errors into its report.

Germany defends monitors' findings

"The German government takes the report by the OSCE and the Council of Europe very seriously, and we expect Turkey to do so," German foreign ministry spokesman Martin Schäfer told reporters in Berlin. Meanwhile, government spokeswoman Ulrike Demmer said: "We will follow closely how Turkey behaves on this. From the German government's point of view, Turkey must ... clear up the questions that have been raised."

Turkish media reported that the YSK made the decision to accept unstamped ballots within about an hour after the AK Party filed a petition on election day. The unstamped ballots and envelopes were then stamped and counted, making it impossible to identify which ballots were in question.

The opposition parties' request to completely annul the results will be decided at the national level by the YSK. The electoral body has already defended its decision on its website, making the odds of annulment low.

The head of the electoral commission, Sadi Guven, has also so far defended the decision but said he would evaluate requests to annul the vote.

rs, cw/msh (AFP, AP, Reuters, dpa)