At least seven people were killed in an explosion at a rally for Ethiopian Prime Minister Abiy Ahmed, who escaped unharmed.
An organiser of the huge rally in the capital Addis Ababa said a member of the crowd had tried to throw a grenade at the stage.
"The prime minster was the target," Seyoum Teshome said. "An individual tried to hurl the grenade toward a stage where the prime minister was sitting but was held back by the crowd."
Mr Seyoum said three suspects, two men and a woman, were immediately arrested.
Minutes after being rushed to safety, Mr Abiy said in a televised address to the nation that "a few people" had been killed and others injured. He called the blast a "well-orchestrated attack" that failed.
"Love always wins," he said. "Killing others is a defeat. To those who tried to divide us, I want to tell you that you have not succeeded."
The blast occurred shortly after Mr Abiy, 42, had addressed the tens of thousands of people who had gathered in Meskel Square to show support for the sweeping reforms introduced since he took office in April.