The Peoples' Democratic Party (HDP) identified the victim as Deniz Poyraz, a party official in the western province of Izmir where the morning attack occurred.

The party's national co-leader said a planned meeting of 40 officials at the office had been called off for unrelated reasons moments before the attack.

"The plan here was clear," HDP co-leader Mithat Sancar told reporters. "What they wanted was a massacre."

The HDP is the Turkish parliament's third-largest party, supporting liberal causes and espousing leftist economic views.

But President Recep Tayyip Erdogan views it as the political front of outlawed Kurdish militants who have been waging a decades-long insurgency that has claimed tens of thousands lives.

Erdogan frequently makes fiery speeches denouncing the party, which has seen scores of its members detained. The government's attempt to use the courts to disband it has drawn sharp rebukes in Europe and the United States.

The HDP said the rhetoric coming from Erdogan's ruling party was the "instigator of this brutal attack".

"After months of attacks by the ruling party and the interior ministry, and families being told to organise outside our provincial offices to cause provocation, there was an attack this morning at 10:30 am on our Izmir province building," it said in a statement.

The ruling AKP party's official spokesman called the incident "a provocation aimed at disturbing (Turkey's) peace and security".

"We condemn the attack and the murder that occurred at the HDP Izmir office," AKP spokesman Omer Celik said.

- 'Served in Syria' -

The Izmir governor's office said the presumed assailant -- who had "resigned as a health worker" -- was in custody.

Local media reports said the suspect claimed to have been posted to Syria, sharing pictures on social media of him posing with Kalashnikov rifles.

Hundreds of HDP members and supporters staged a hastily-arranged demonstration in Istanbul after the attack, with a similar protest reported in the mostly Kurdish southeastern city of Diyarbakir.

The shooting came after a top prosecutor refiled a case to the Constitutional Court in an effort to shut down the party and bar 451 of its members from being involved in politics.

The top court is expected to decide Monday whether to take on the case.

The HDP denies formal ties to Kurdistan Workers Party (PKK) militants, who are recognised as a terrorist organisation by Turkey and much of the West.

The party sees itself as a victim of a sweeping crackdown in which tens of thousands were jailed or stripped of their government jobs after Erdogan survived a failed military putsch in 2016.

Multiple legal cases against HDP officials are winding their way through the courts.

The latest saw 108 HDP members and former leaders put on trial for their alleged roles in deadly 2014 protests that erupted as extremists from the Islamic State group overran the Syrian town of Kobane.

Former party co-leader Selahattin Demirtas -- who twice challenged Erdogan in elections -- has been held since November 2016 and could spend the rest of his life behind bars if convicted of various terror charges.