The blast hit a police control post on the highway leading southeast from Diyarbakir to the city of Batman, the local governor's office said in a statement, adding that 25 people were wounded including five police officers.
Television images showed the bombing had turned the three- storey police building to tangled rubble. Locals were also inspecting the massive trench where the ground had been blown out.
The latest bombing came after at least eight people, mostly civilians, were killed on Wednesday in two separate attacks blamed on PKK militants in Turkey's southeast.
The PKK has kept up attacks after the July 15 failed coup during which a rogue military faction tried to oust President Recep Tayyip Erdogan from power.
The government has vowed there will be no let-up in the fight against the PKK even in the wake of the coup.
Over 40,000 people have been killed since the PKK first took up arms in 1984. It is proscribed as a terrorist group by Turkey, the European Union and the United States.