The blast hit a police traffic control post on the highway leading southeast from Diyarbakir to the city of Batman, killing the six victims, Deputy Prime Minister Numan Kurtulmus told reporters in Ankara.
The local governor's office earlier said in a statement that 25 people were wounded including five police officers. One of those killed was a child.
An initial toll had said three people were killed.
An AFP photographer at the scene saw the bombing had turned the three-storey police building into tangled rubble with rescue workers searching the debris with diggers for corpses.
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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.
It also came on the day seen as marking the 32nd anniversary of the launch of the PKK's armed rebellion by radical Kurdish nationalist Marxists led by their now jailed leader Abdullah Ocalan.
Hundreds of members of the Turkish security forces have been killed by the PKK in attacks since the collapse of a two-year ceasefire in July last year.
The PKK has kept up attacks after the July 15 failed coup that tried to oust President Recep Tayyip Erdogan from power, which authorities blamed on US-based preacher Fethullah Gulen.
The government has vowed there will be no let-up in the fight against the PKK even in the wake of the coup.
The Turkish army's hierarchy has been badly hit in the purge since the coup, with top generals accused of complicity in the plot.