A "bomb attack" on a rally in support of Ethiopian Prime Minister Abiy Ahmed injured nearly 30 people Sunday, a police official said, in the latest sign of instability ahead of elections in August.The incident occurred in the town of Ambo, located roughly 100 kilometres (60 miles) west of the capital, Addis Ababa."The bomb attack on a rally for Dr. Abiy has injured 29 people, of whom 28 have been treated and sent home," Arasa Merdasa, the top police official in Ethiopia's Oromia region, where Ambo is located, told the state-run Ethiopian News Agency."Police have arrested six people who are suspected in the attack," Arasa said. Ethiopia's electoral board has scheduled landmark national polls for August 29.READ | Did Ethiopia's Abiy Ahmed get the Nobel Prize too soon?Opposition parties and civil society organisations have questioned whether the elections will be peaceful and credible, citing persistent ethnic violence since Abiy was appointed in 2018 following several years of anti-government protests. The formal campaign period begins in May. Abiy did not attend Sunday's rally, which was organised by officials in Ambo. Abiy, the winner of last year's Nobel Peace Prize, hopes the elections will secure him a mandate to continue with an ambitious agenda of political and economic reforms.Arasa said Sunday's attack was believed to be the work of the Oromo Liberation Army (OLA), the breakaway armed wing of the Oromo Liberation Front, an opposition party.Officials have also blamed the OLA for the assassination on Friday of the top security official in Burayu, another Oromia town located on the outskirts of Addis Ababa.That attack left three other people injured, and police "vowed to hunt down" those responsible, state-affiliated Fana Broadcasting Corporate reported.Arasa declined to answer questions about the latest violence in Oromia when contacted Sunday, referring an AFP reporter to the Ethiopian News Agency report.