The rally was held on Sunday evening at Tilauthu three days after Congress president and UPA chairperson Sonia Gandhi campaigned for Kumar in Sasaram.
Congress and RJD are fighting separately the by-poll to five assembly seats and Prasad had been kept away by Congress from rallies of Sonia Gandhi and Rahul Gandhi in Aurangabad last week fuelling the rumours.
He told the poll rally that the unity among secular parties Congress and RJD would keep away the communal forces from power.
Prasad described the BJP and RSS as communal forces who were posing a "threat" to the country and said he would stop Narendra Modi this time just as he had stalled BJP stalwart L K Advani's rathayatra in the past.
On his political rivals, he said, "JD(U) and BJP entered into love marriage 17 years ago to keep me out of power in Bihar. This has resulted in divorce and now that I am out of jail, their plans of capturing power have failed."
Meira Kumar in her address highlighted achievements of UPA government and vowed to take forward the development works in the constituency.
She told Prasad in Bhojpuri that both Congress and RJD workers are working unitedly in Sasaram.
Sasaram (SC) seat will go to polls on April 10.
Meanwhile, Meira Kumar's estranged neice Medhavi Kiri, whose nomination for the Sasaram seat was rejected on technical grounds, is campaigning for JD(U) candidate K P Ramaiah.
Medhavi, who is a granddaughter of dalit leader Jagjivan Ram, has been campaining in Kaimur region for Ramaiah to help him defeat Meira Kumar on her home turf.