Demirbas was late for our interview in a café garden in the historic centre of Diyarbakir because he had to visit the police station, a task he is well acquainted with.
He is the former mayor of this area, a high-profile position in this restive region, and he has been arrested three times - once in 2009, when he was held in jail for eight months, and twice this year.
“I was also arrested in 2009 and I was in jail for about eight months and they released me at the time due to my health," he explains. "Also they arrested me in 2015, in August, despite me having reports in my hands in regards to my health. Of course, later on, once conditions were really serious, they released me.”
But this time he doesn't know why.
“Now I don’t know why I was arrested because they declared my case secret, so neither I nor my lawyers know why I was arrested. After public support and, of course, due to my health condition, I was released.”
As we spoke, Demirbas received the news that a ban on his travelling to Turkey's largest city, Istanbul, had been lifted.
In all, 18 mayors associated with the HDP have been arrested this year, Demirbas says, and so have about 1,000 of the party's members.
After failing to form a coalition in June, President Recep Tayyip Erdogan's government broke off peace talks with the PKK, claiming that the guerrillas had broken their truce with the Turkish state.
That led to a military offensive, which was met by armed action by young Kurds, who declared autonomous zones in some areas, including part of Diyarbakir.
The government's opponents say the offensive and the clampdown on the HDP were a ploy to boost support for the AKP.
But it has backfired, according to Demirbas.
“On the contrary," he says. "Once people saw the pressure on the HDP, they understood that this regime is very totalitarian, dictatorial and they have also a fundamentalist belief in themselves.”
When the Islamic State armed group seized swathes of territory in neighbouring Iraq and Syria, Turkey agreed to join the US-led campaign of air strikes against it.
But it also bombed positions of the PKK and its Syrian allies, the YPG, who have waged the most effective campaign of resistance to the IS.
The Turkish government is furious because the YPG has established an autonomous zone in Rojava and they don't want that to spread, says Demirbas.
“In Rojava the Syrian Kurdish peope making democratic autonomy by themselves is not something that is wanted by this regime. Because they don’t want this, everyone knows this regime is supporting [the Al-Qaeda-affiliated] Al Nusra and IS. Many people are coming through Turkey from Europe and the rest of the world, everyone knows that, they are joining this group. Despite the decision that they were going to bomb and make operations against IS with coalition forces, Turkey has attacked the PKK but not attacked IS.”
Opinion polls show that Sunday's election is likely to produce a similar result to June's, meaning that there must be new attempts to form a coalition.
Surprisingly for such a radical party, the HDP is ready to support a coalition of the AKP and the secular nationalist Republican People's Party, the CHP.
“For me the best thing would be an AKP-HDP coalition," Demirbas explains. "Maybe with a serious democratic programme and serious work, they can successfully make Turkey into a democratic regime. A civil and democratic constitution can be established. Work like this can be supported by HDP from outside and they will support it.”
Another deal could be struck, however.
The AKP might form a coalition with the right-wing secular Nationalist Action Party, the MHP, which is even more fervently anti-PKK than Erdogan.
That would be "like civil war", Demirbas declares ... a gloomy prophecy for a country that has seen more than its share of violence over the years.