"Someone tried to hurl it to the stage where the prime minister was", Seyoum Teshome, a member of the organizing committee, told Reuters.
It occurred as he had just finished speaking to tens of thousands of supporters in Meskel Square in Addis Ababa, the Associated Press reported.
Ethiopia's announcement on the peace deal set off a number of protests by residents in the northern Tigray region bordering Eritrea.
Ahmed was sworn in as prime minister in April on a platform to unite Ethiopians and improve relations with neighboring Eritrea.
Eritrea, the country that has been hosting anti-TPLF rebel groups, announced today that it is sending delegation to Ethiopia to figure out next step for the peace talk proposed by prime minister Abiy Ahmed's government. Police have said more than 100, while Abiy's chief of staff cites reports from police and hospitals in saying 83 people were hurt, with six in critical condition.
The movement rated the prime minister's speech at the Ethiopian parliament on Monday as encouraging and positive.
After the blast, dozens of people stormed the stage, hurling various objects at police, shouting: "Woyane down, down", and "Woyane thief", using a pejorative term for the government, the AFP correspondent said.
He said it was a "well-orchestrated attack" but it had failed.
Ethiopia's new Prime Minister Abiy Ahmed escaped a grenade attack on Saturday (Jun 23) at a rally in the capital, although 132 people were wounded and one person killed, officials and witnesses said.
He later confirmed people had been injured in the blast.
President Kenyatta urged Prime Minister Abiy to continue the task of uniting the Ethiopian people, ensuring the stability of the country and sustained growth of its economy.
Mr Ahmed was taken away by security personnel immediately after his speech.
Mr Abiy has been causing waves after announcing the release of tens of thousands of prisoners, the opening of state-owned companies to private investment and the unconditional embrace of a peace deal with rival Eritrea.
"Ethiopia will be on top again, and the foundations will be love, unity and inclusivity", he declared on Saturday, dressed in a green T-shirt and a hat.
The United States has been among other nations expressing support for the changes in the country, a key security ally.