Ten Turkish soldiers and eight civilians were killed when suspected Kurdish militants detonated a five-tonne truck bomb near a military outpost in the country's southeast.
Another 27 people, including 11 soldiers, were wounded in Sunday morning's blast which hit the Durak gendarmerie station in one of the most deadly attacks in the region of recent times.
The mountainous Hakkari province, where the attack occurred, lies near the border with Iraq and Iran and is one of the main flashpoint areas in a conflict that has pitted Turkey's army against the militant Kurdistan Workers Party (PKK) for 32 years.
A small truck approached the vehicle checkpoint and ignored an order to stop, prompting gendarmerie troops to open fire, the Hakkari governor's office said.
A bomb in the vehicle was detonated, which Prime Minister Binali Yildirim told reporters contained some five tonnes of explosives.
President Tayyip Erdogan vowed to put an end to PKK attacks as he condemned the bombing, accusing the group of acting on behalf of "dark forces that had designs in Syria and Iraq".
The army has "neutralised" 387 PKK fighters in Hakkari alone since August 4, state-run Anadolu Agency cited military sources as saying.
The governor's office said extensive air-backed operations were being conducted by commando units in the area to capture PKK militants, who were believed to have opened fire in the run-up to the attack to distract soldiers at the checkpoint.
Authorities were on high alert for possible attacks on Sunday, 18 years to the day since PKK leader Abdullah Ocalan fled Syria before being captured by Turkish special forces in February the following year.
He has since been in prison on an island near Istanbul.