By Ece Toksabay and Gulsen Solaker
ANKARA (Reuters) - The head of Turkey's pro-Kurdish opposition accused President Tayyip Erdogan of trying to create a "constitutional dictatorship" by pushing for an executive presidential system, and of fostering a climate of fear to win an election landslide.
Selahattin Demirtas, who co-leads the Peoples' Democratic Party (HDP), told Reuters he believed the government could call a referendum as soon as this time next year on a new constitution that would transform what has been a largely ceremonial presidency.
A senior member of the AK Party, which Erdogan founded, denied the charges of fear mongering, saying that on the contrary it had suffered while trying to campaign in the predominantly Kurdish southeast of Turkey.
In the second election to be held this year, the AKP last week regained the parliamentary majority which it lost in June, winning 317 of 550 seats - only 13 short of the number needed to call such a referendum.
Demirtas stressed the HDP would oppose the plan of putting more power in the hands of the presidency, which Erdogan has held since stepping up from the prime ministership last year.
"We would have to lose our minds to agree to this," he said in an interview conducted on Sunday. "Erdogan's plan for the executive presidential system is not a model for an executive presidency but one-man rule, a constitutional dictatorship that merges all authority into a single hand."
Erdogan's spokesman said after the election on Nov. 1 that an issue such as changing the presidential system couldn't be decided without the nation's support and if a referendum were needed, then one would be held.
Speaking at the HDP headquarters, Demirtas said next summer might be too early for a referendum but it could happen as soon as autumn 2016.
The HDP stunned pundits and pollsters in June when it won more than 13 percent of the vote, reaching beyond its Kurdish base to secular leftists and others put off by years of rule by Erdogan and the AKP.
But in the re-run, held after coalition negotiations failed, it barely scraped over the 10 percent threshold required to enter parliament, taking 59 seats, or 21 fewer than in June.
The rise in AKP support appeared to have been motivated by renewed fighting in the southeast between the security forces and Kurdistan Workers Party (PKK) militants after a ceasefire collapsed in July. Hundreds have since died.
Conservative Kurds and liberal Turks who blame the PKK for the unrest turned their backs on the HDP.
Demirtas blamed an atmosphere created by the "presidential palace" for the fall in support for his party.
"In order to keep the HDP under the threshold, the Palace decided on a war campaign that put attacks against our party in the center," he said. "The AK Party and the Palace saw it was possible to increase their support by frightening people. They are going to use it more often now, as they saw that it actually worked."
Suicide bombers have targeted HDP sympathizers twice since the June election, including in Ankara last month when more than 100 people were killed in Turkey's deadliest such attack.
AKP Deputy Chairman Ayhan Sefer Ustun rejected the accusations from Demirtas, saying his party's supporters had been threatened in Kurdish areas before the election.
"It's not we who created the atmosphere of fear, but the PKK and the groups linked to it. The HDP is the one who benefits from such an atmosphere," he told Reuters. "The AK Party could not run an election campaign in the southeast due to this atmosphere."
Erdogan, who will host a G20 summit in a week's time, has said the election outcome was a vote for stability and a message to the PKK and its allies that violence could not coexist with democracy.
With President Barack Obama expected to attend, Washington has said it is deeply concerned that Turkish media and journalists were subject to pressure during the campaign.
Erdogan has consistently portrayed criticism of his leadership as part of a foreign-backed effort to belittle him and undermine Turkey's influence in the region.
Demirtas said he expected the government would eventually return to peace talks with the PKK.
"The negotiations between Turkey and the PKK will eventually restart, you can't keep fighting forever," he said. "I can't name the exact date but I think parties will return to the peace process in the medium term."
(Additional reporting by Ercan Gurses in Ankara; Writing by Ece Toksabay; Editing by David Dolan and David Stamp)