ADDIS ABABA, Ethiopia — An attacker dressed in a police uniform tossed a grenade at a rally for Ethiopia’s new prime minister on Saturday, killing at least one person and setting off a stampede as panicked people rushed to safety, according to officials, witnesses and the state broadcaster.

On Sunday, Ethiopia’s health minister, Amir Aman, said another person had died in intensive care.

Witnesses said the attacker, dressed in a police uniform, struck shortly after Prime Minister Abiy Ahmed gave a speech at Meskel Square in Addis Ababa at a large public rally. More than 150 people were injured, according to a government official.

The rally organizer said the prime minister had been targeted, although he was not injured. The deputy police commissioner for the capital, Addis Ababa, and eight other police and security officials were arrested on suspicion of complicity in the attack, state television quoted the country’s minister of communications as saying.

Mr. Abiy — a 41-year-old former soldier, minister of science and technology, and vice president of the Oromia region — took office in April, pulling Ethiopia back from the brink of a political implosion. The country, rocked in recent years by violent protests, had been in a state of emergency since the previous prime minister’s resignation in February.