Six people were injured in the blast at the office of the People's Democratic Party (HDP) in the southern city of Adana caused by a suspect parcel, three of them seriously, a party official told AFP.
A bouquet of flowers sent to the party's office in the nearby city of Mersin also exploded, the official said.
Video footage showed several people with bloodied faces.
The HDP's co-chairman Selahattin Demirtas was due to address a rally in Mersin later in the day but the party said there was no suggestion of him cancelling the event.
The government immediately condemned the attack, with Prime Minister Ahmet Davutoglu pledging to bring the perpetrators to justice.
"I strongly condemn this attack," Davutoglu told a rally in the city of Karaman in central Turkey.
Culture and Tourism Minister Omer Celik announced on Twitter that a special unit involving the police, gendarmerie and the intelligence agency was established to find out culprits of the attacks which he said took an aim at "the HDP, all political parties and Turkey's election process."