The four-day Chhath Puja began on Sunday with people visiting river banks early in the morning for a ritualistic bath, before preparing a traditional meal of boiled rice and pumpkin.
Chhath is celebrated on the sixth day after Diwali and is dedicated to the Sun. It is one of the most popular festivals in Bihar.
During the festival, women observe a continuous fast for 36 hours. Devotees offer wheat, milk, sugarcane, bananas, and coconuts while praying to the Sun.
The ritualistic bath known as the 'Nahai-Khai' will be followed by 'Kharna' on Tuesday, when 'Kheer' will be cooked and distributed among neighbours, friends, and relatives.
On Wednesday evening, devotees will offer prayers called 'argya' to the setting Sun and follow this up by offering prayers to the rising sun on Thursday morning.
Meanwhile, district magistrates have been directed by Chief Minister Nitish Kumar to give top priority to safety and security of devotees.
In Patna, about 200 divers have been deployed to handle any emergency near the river banks and over 5,000 policemen have been deployed to maintain law and order.
Over the years, Chhath has come to be closely identified on the lines of Bihu in Assam, Pongal in Tamil Nadu and Ganesh Chaturthi in Maharashtra.