All roads virtually led to the Gandhi Maidan as participants marched to the venue waving green flags of the JD-U and RJD and the tricolour of the Congress.
Senior Superintendent of Police Manu Maharaj said nearly 5,000 police personnel had been deployed in and around the sprawling ground.
Police also set up 10 watch towers and 13 door frame metal detectors. Bomb disposal and anti-terrorist squads too have been deployed, he said.
JD-U leader and Bihar Chief Minister Nitish Kumar and Rashtriya Janata Dal chief Lalu Prasad will address the rally.
Congress president Sonia Gandhi, JD-U president Sharad Yadav and Samajwadi Party leader Satpal Yadav, a brother of party chief Mulayam Singh Yadav, will also attend the rally.
This is the first big show of unity a year after Nitish Kumar and Lalu Prasad joined hands in a bid to defeat the BJP-led alliance in assembly elections in the state.