The Met Office has predicted more rain in the next 24 hours and the state government has issued a 'red alert'.
The unexpected 'heavy to extremely heavy rain' has so far claimed the lives of over 24 people - mostly drowned, across the state, officials of disaster management department said here.
The water level in major rivers including Ganga, Koshi, Gandak, Bagmati, Mahananda, are on the rise, threatening to breach embankments at many places.
The water resources department has alerted the concerned officials and asked the district authorities to take measures to deal with any possibility.
The Centre has already deployed teams of National Disaster Response Force and State Disaster Response Force for rescue and relief operations in vulnerable places.
According to weather department officials, state capital Patna has received 151 mm rain since Saturday a record in recent years.
Patna is now facing a flood-like situation with water is all around. People have been forced to stay indoors as most of the roads in the narrow lanes were water-logged.
Boats are being deployed in several localities to help people.
"This is the first time I have seen boats plying on the water-logged roads in Patna, it is new for us," Sanket Jha, a college student of Rajendra Nagar colony, said.
"Entire Patna is water-logged because of choked drainage. It has caused havoc in residential localities and exposed the Patna Municipal Corporation," Lakhender Mahto, a retired school teacher, told here.
The rain has also affected the movement of trains, road traffic as well as flight operations. Twelve long route trains and several passenger trains have been cancelled.
There were reports of damage to the National Highways at several places in north Bihar districts.
The flood water has also entered houses, shops and hospitals at many places in the state.
According to reports, water entered the residence of Bihar Deputy Chief Minister Sushil Kumar Modi in Rajendra Nagar.
Water also made its way to the residence former Chief Minister Satender Narayan Singh on Boring road, BJP MP Rajeev Pratap Rudy, and former Chief Minister Jitan Ram Manjhi in Patna.
( With inputs from IANS )