The explosions have apparently targeted central train station as activists gathered for a peace rally organized by leftist and pro-Kurdish groups.
TV footage shows scenes of panic and people lying on the ground covered in blood, amid protest banners.
The pro-Kurdish HDP party was among those attending the rally.
Meanwhile, Turkish government officials described the blast as a “terrorist attack”, and said they were investigating claims that a suicide bomber was responsible.
The blast occurred ahead of a planned peace march to protest against the conflict between the state and Kurdish militants in southeast Turkey.