The co-chair of Turkey's main Kurdish party has accused President Recep Tayyip Erdogan of wanting to set up "a constitutional dictatorship", vowing his movement will strongly oppose moves to impose one-man rule.

Selahattin Demirtas of the People's Democratic Party (HDP) told AFP that his party hoped to turn Erdogan's political calculations "upside down" in June 7 legislative polls.

Erdogan's ruling Islamic-rooted party had been relying on Kurdish support to push through changes to the country's constitution.

The controversy comes amid indications of delays in the peace process to end a decades-long insurgency by Kurdish rebels, with Demirtas saying talks had not been helped by recent comments made by Erdogan.

"Mr President is trying to create a constitutional dictatorship by collecting all the power for himself," Demirtas said at his party's headquarters in Ankara.

Erdogan, elected head of state last year after more than a decade as prime minister, wants to rewrite the constitution to create a executive-style presidency.

Demirtas said that Erdogan was already riding roughshod over the existing constitution in the run-up to the polls by not severing his links with the ruling Justice and Development Party (AKP) as required of the president.

"The constitution is currently suspended in Turkey," he said.

"Turkey will face big challenges if someone who does not recognise the constitution today brings in one-man rule under the pretext of presidential system tomorrow."

- 'Kurdish Obama' -

Erdogan has appealed to his supporters to help elect 400 AKP lawmakers in June's election, which would allow the party free-rein to change the constitution in the 550-seat parliament.

But the party needs votes from the Kurds, who make up an estimated 20 percent of Turkey's population and are its biggest minority.

Should the HDP pass the 10 percent minimum threshold of votes required to enter parliament, the AKP may not find the majority to change the basic law.

Support for the HDP is currently hovering at around 10 percent, according to polls.

"If the HDP passes the 10 percent threshold, many political parties' calculations will be turned upside down," Demirtas said.

Demirtas came third in 2014 presidential polls with just under 10 percent of the vote. His good looks and impassioned rhetoric earned him the nickmame "Kurt Obama" ("The Kurdish Obama"), in reference to the US leader.

Opposition parties claim that the AKP and the HDP struck a secret deal in which the Kurdish party would support presidential system in return for reforms -- a claim vehemently rejected by Demirtas.

"We have neither an open nor secret deal with the AKP for the pre- or post-election period," he said.

Demirtas said he also hopes his party will win support from non-Kurdish voters who back his liberal, secular and pro-women policies and who are tired of Erdogan's "polarisation" of the country.

- Stalled peace process -

Erdogan is credited with improving the rights of Turkey's Kurdish minority and helping launch a peace process aimed at ending the armed struggle by Kurdistan Workers Party (PKK) militants which has claimed tens of thousands of lives.

But he infuriated Kurds last month by saying there was no longer a "Kurdish problem" in Turkey, a comment seen by some as a blatant attempt to snare Turkish nationalist votes in the election.

He also engaged in a rare dispute with the government over the handling of peace talks with the PKK.

"Mr President's declarations have clearly troubled the (peace) process," said Demirtas.

He said that while the president and the government were in agreement on policy there was a "difference in tactical approach ahead of the elections".

Jailed Kurdish rebel leader Abdullah Ocalan, who is serving a life sentence on a prison island in the Sea of Marmara, began clandestine talks with Turkish state in 2012 to end the rebellion.

The PKK has largely observed a ceasefire since then but attempts to find a permanent deal have stalled over the issue of the withdrawal of PKK fighters and weaponry from Turkey.

Erdogan is pressuring the PKK to disarm as a pre-condition of a final peace deal, a demand that appears to pose a stumbling block in talks.

Demirtas declined to provide a timeline about a possible deal, saying that it largely depended on the steps to be taken by the government.

"The faster the steps the government takes, the faster a solution will be reached. Both Mr Ocalan and PKK officials declared that they are ready to lay down arms," he said.

Demirtas said the PKK and Ankara agreed to take mutual steps in which reforms and disarmament would come simultaneously but he said his party would not object if the PKK and Ocalan accepted disarmament beforehand.

"But we know that negotiations are not proceeding in that direction," he said.