Incessant rain in the 15 districts spread across upper, central and lower Assam caused the first wave of floods, official sources said.
Among the worst affected districts are Dhubri, Nalbari, Barpeta, Morigaon, Jorhat, Dibrugarh and Tinsukia.
While boats have been engaged to transport affected people to safer places, the district administration has been keeping round-the-clock vigil.
Incessant rain has resulted in the alarming rise in water level of the Brahmaputra and its tributaries in the past few days.
The administration is also keeping a watch on flood-prone Kaziranga National Park, home to the highly endangered one horned rhino.
The park, a world heritage site, is currently closed for visitors due to the rainy season.
