By Tuvan Gumrukcu and Orhan Coskun
ANKARA (Reuters) - Ten days after it annulled Turkey's most dramatic election upset in years, the country's electoral authority faces a barrage of questions from opposition parties who say there was no legal basis to cancel the vote.
The High Election Board said irregularities affected the outcome of the March 31 mayoral election in Istanbul, when the main opposition Republican People's Party (CHP) narrowly defeated President Tayyip Erdogan's AK Party (AKP).
It ruled that the vote in Turkey's largest city and commercial hub, the biggest prize in the nationwide local elections, be re-run next month because some polling officials were not civil servants, as required by voting regulations.
The decision to reverse Erdogan's rare election setback was described by Turkey's Western allies as incomprehensible. Critics said one of the last checks on his ever-tighter hold on power had suffered a damaging blow.
"The most fundamental value of our political tradition is that the last word belongs to the national will, which is manifested in the ballot box," Erdogan's erstwhile ally Ahmet Davutoglu, a former AKP prime minister, said last week.
"The annulment decision has opened the way to damaging these fundamental values of ours."
Erdogan has ruled Turkey since 2003, first as prime minister and then as president, winning more than a dozen elections.
He remains Turkey's dominant politician, but economic recession and a slump in the lira have eroded support for his Islamist-rooted AKP. The CHP argues that by refusing to accept any loss of power, he is dragging Turkey deeper into authoritarianism with the help of increasingly co-opted institutions such as the High Election Board (YSK).
"A game where those who come with elections don't leave with elections was approved by a gang influencing the YSK," CHP spokesman Faik Oztrak said last week. "A coup was carried out on the ballots, which are the last bastion of democracy."
Erdogan's party says it provided concrete evidence of wrongdoing in the electoral process and the election board acted solely on the information available. "The power of the ruling party was never used on the YSK," an AKP official told Reuters.
The AKP described the irregularities in the mayoral vote as "organised crime" which affected the outcome. The AKP has said the re-run of the mayoral election was aimed at ensuring the public will was reflected in the ballot.
The YSK decision was passed by a 7-4 majority. Its 11 members, all judges, are chosen by Turkey's two highest courts whose members are selected by a judicial council appointed partly by parliament and partly by the president.
The election board has yet to publish a detailed explanation for its decision. "When the reasoning is finished, we'll share it," YSK head Sadi Guven, who was part of the dissenting minority, told reporters on Wednesday.
In a May 6 statement, it said the ruling was based on the fact that some polling stations in Istanbul were "formed illegally by the district electoral board, and this issue impacted the results of the elections".
A day after the announcement, a Turkish lawyers group said the irregularities cited in the YSK ruling should have been challenged before the vote, when they were apparent.
The Union of Turkish Bar Associations said the YSK had also failed to explain how those violations had changed the result of the election, and the ruling contradicted several previous YSK decisions.
Two years ago the YSK angered Erdogan's opponents when it ruled, in the midst of voting on a tightly fought referendum to grant the president sweeping executive powers, that unstamped ballot papers would be accepted - a decision which the bar associations said lifted a safeguard against voter fraud.
Aylin Ozgul Kirmizioglu, one of the four party representatives who are allowed to observe YSK meetings, said she was stunned to hear one of the judges first support cancelling and re-running the election.
"When I heard the first annulment vote, I had the shock of my life," Kirmizioglu, from the opposition Iyi (Good) Party which was allied to the CHP, said. "I was truly very surprised and we, as the representatives, all looked at each other to see if we had heard right."
Kirmizioglu said it was illogical for the YSK to rule the mayoral election invalid when three other votes for local councils and administrators submitted in the same envelopes at the same polling stations on the same day were deemed valid.
The AKP argued it was reasonable to focus on the mayoral election because the victory margin - 13,000 votes in a turnout of nearly 9 million - was so thin.
Kirmizioglu said: "It's very wrong to expect the public to understand this, since even we as lawyers don't get it.
"If a legal rule is against logic, then it's not legal".
(Editing by Dominic Evans and Janet Lawrence)