Mohammad Shahabuddin, facing 35 cases including murder, was released from jail on September 11.
The BJP-led National Democratic Alliance (NDA) on Friday held protest demonstrations in Siwan, the home district and stronghold of Mohammad Shahabuddin after the former Rashtriya Janata Dal (RJD) MP from Bhagalpur was released from jail after 11 years.
"We are now holding a 'mahadharna' (mega protest rallly) in Siwan to protest against the release of Shahabuddin," senior Bharatiya Janata Party (BJP) leader Prem Kumar said.
The RJD leader faces 35 criminal cases, including murder, extortion and kidnapping. He was only convicted in seven cases.
The Siwan protest comes after NDA leaders and workers staged rallies at all district headquarters across the state. Siwan is considered Mr Shahabuddin's stronghold. He is also known as the "Sultan of Siwan".
Mr Kumar said the BJP and its allies would continue to raise this issue to expose the state government's alleged double standards on the rule of law.
"We will also approach President Pranab Mukherjee in this matter soon," he said.
Senior BJP leader and former Deputy Chief Minister Sushil Kumar Modi reiterated his demand to the state government to send Mr Shahabuddin back to the jail.
Mr Shahabuddin was granted bail by the Patna High Court on September 10 in a case involving the murder of a witness in the killing of two brothers in Siwan.