The event at Jagannathpur Maidan was attended by Union Home Minister Amit Shah and party president J P Nadda.
The BJP has made all out efforts to make the merger and Marandi's re-joining a grand event.
The first BJP Chief Minister of Jharkhand was being wooed as part of the attempt to revive the party's presence in the state among tribals, which of late seems to be turning away from it.
Marandi had on February 11 announced JVM-P's merger with the BJP, 14 years after he had quit the saffron party and floated his outfit.
The JVM-P has three MLAs, but two of them--Bandhu Tirkey and Pradip Yadav- have rebelled and Marandi had decided to expel them from the party.
Marandi will be the lone legislator of the party after doors were shown to the two others.
Reports say Marandi is likely to be elected as the Legislative party leader of the BJP, a post that is lying vacant.
The 81 assembly constituencies of Jharkhand went to polls on in five phases. BJP has 25 legislators presently in the 81-member Jharkhand Assembly. It was unseated by the opposition alliance of JMM- Congress-RJD in the state polls in December last year. Marandi won the assembly elections from Dhanwar constituency.
(With additional information from Agencies)