Turkey’s high election board has rejected formal calls by the country’s main opposition parties to annul the result of a referendum that will grant Recep Tayyip Erdoğan sweeping new powers as president.

Voters narrowly approved a set of constitutional reforms that will transform the country from a parliamentary democracy into a presidential republic, concentrating power in the hands of Erdoğan, who will be able to run for two more terms and potentially govern until 2029.

The two main opposition parties – the Republican People’s party (CHP) and the People’s Democratic party (HDP) – had lodged formal complaints calling for the annulment of the result, citing a controversial last-minute decision by the board to allow the counting of possibly hundreds of thousands of unstamped ballots. The constitutional amendments passed with a margin of just over a million votes.

International observers had said the decision to count the ballots “contradicted the law” and removed a safeguard against fraud.

But the board said in a statement on Wednesday that its members had rejected all of the appeals in a 10-1 vote. It cited precedents in which it had accepted unstamped ballots as valid due to the incompetence or lack of training of local ballot box officers who had failed to stamp them in time for elections.

The decision upholds the result of the landmark referendum, which has polarised Turkey in the last few months and highlighted the splits in a society that is still reeling from a coup attempt last year and repeated terror attacks by Islamic State and Kurdish separatists.

The amendments will empower Erdoğan, the most influential politician in Turkey since the country’s founding on the ruins of the Ottoman empire by Mustafa Kemal Atatürk. They will remove oversight powers from parliament and allow Erdoğan to return to the leadership of the party he founded, the Justice and Development party (AK), granting him control over lawmakers who will run in elections as well as a high percentage of judicial appointments.

The CHP, the largest opposition party in parliament, said on Wednesday it would never accept the legitimacy of the result of the referendum, raising the spectre of even withdrawing from the legislature.

“We don’t and won’t recognise this referendum result,” the party’s spokeswoman, Selin Böke, said, adding that the CHP would exercise all its democratic rights including the possibility of “seceding” from parliament. “The referendum ... is null and void,” she said.

Opposition parties have argued that the constitutional amendments do not provide for any checks and balances on Erdoğan’s power, setting the stage for one-man rule in Turkey.

The government fanned concerns over the curtailing of freedoms by launching a campaign of arrests on Wednesday against protesters who demonstrated against the results of the referendum. Local media reported that 38 people were detained in raids by anti-terror police that constitute the first political arrests since the vote.

Turkey’s prime minister, Binali Yıldırım, had earlier in the day warned the opposition against street protests over the referendum.

Tens of thousands of civil servants, academics, police and military officials have been arrested in a wide-ranging crackdown since the coup attempt, and dozens of journalists are also facing trials. International observers said the vote had been carried out in a political environment in which fundamental rights and freedoms had been curtailed.

Turkey has been in a state of emergency since the coup attempt. The state of emergency was renewed for three months this week.