CM of Manipur N Biren Singh, CM of Arunachal Pradesh Pema Khandu, CM of Assam Sarbananda Sonowal and CM of Meghalaya Conrad Sangma were present at the oath ceremony in Kohima.
The venue holds significance as the announcement of Nagaland's statehood on December 1, 1963 was made from here by the then president Sarvepalli Radhakrishnan.
The Nationalist Democratic Progressive Party (NDPP), in alliance with the BJP, will take over the reins of the north-eastern state with the support of 32 MLAs.
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The alliance has chosen senior NDPP leader Rio as the chief minister. Governor P B Acharya administered the oath of office and secrecy to Rio.
The newly-mandated alliance government, comprising 32 elected members of the NDPP, BJP, JD(U) and an Independent, however, decided to take the oath before the public.
Rio will be assuming the charge of Nagaland chief minister for the fourth time. He had served his three previous terms between 2003 and May, 2014.