"To be able to protect the constitutional system, declaring a state of emergency has become necessary", EBC said, quoting a government communique.
"We strongly disagree with the Ethiopian government's decision to impose a state of emergency that includes restrictions on fundamental rights such as assembly and expression, " the statement said.
It did not state how long the emergency will be in effect.
The resignation of Hailemariam, in power since 2012, comes amid protracted anti-government protests and follows a nationwide state of emergency past year. The prisoner release came after protests and a strike in Oromia that urged the government to fulfil its pledge in this regard. The measure led to the arrest of more than 22,000 people before it was lifted in August.
Fegessa also revealed that the state of emergency would further extend its duration if the expected outcomes are not achieved. A stampede at a religious event south-east of the capital that month killed several dozen people.More news: Microsoft's supercharged Windows 10 'Ultimate Performance' takes aim at high-end PCs
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In recent months, internal political wrangling between four coalition partners of EPRDF, Tigray Peoples Liberation Front (TPLF), Amhara National Democratic Movement (ANDM), South Ethiopia People's Democratic Movement and Oromo Peoples Democratic Organization (OPDO) has cast fears over the stability of the nation of about 100 million people. It remains unclear who will then take over.
The United States has responded to the latest unrest by warning its embassy personnel to suspend all travel outside of the capital.
Amid the new political uncertainty, the US statement warns that the state of emergency "undermines recent positive steps toward creating a more inclusive political space".
Befekadu Hailu, a prominent blogger who has been jailed for his writings, urged Ethiopia's government to "carry out genuine reforms, negotiate with legitimate opposition groups and prepare the country for a free and fair election" to solve the unrest.
The new state of emergency will create a group of people with conflicting interests, Befekadu said. "It brings temporary silence but not normalcy".