Authorities at the Patna Medical College Hospital said there were no external injuries on the body.
The BJP has alleged that one of its leaders died after police in Patna fired tear gas shells, used water cannons and resorted to a lathicharge to stop party workers from marching to the Bihar Assembly today. The march was part of a massive protest organised by the party against the “corruption” carried out by the Nitish Kumar-led state government.
While BJP MP and former Bihar deputy chief minister Sushil Kumar Modi claimed the party's Jehanabad district general secretary died due to injuries suffered in the "brutal" police lathicharge, authorities at the Patna Medical College Hospital said there were no external injuries on the body.
The police have also claimed that the lathicharge and other measures became necessary as the protesters threw stones and red chilli powder at them.
"Arrested by Bihar police in Patna. Jehanabad dist GS Vijay Kumar Singh died in brutal police lathi charge," Mr Modi tweeted.
The superintendent of the Patna Medical College Hospital confirmed the BJP leader's death but hospital authorities said he was brought dead and there were no external injuries on his body. They said only a post-mortem will reveal the exact cause of death.
Union Minister of State for Home Affairs Nityanand Rai said, "Vijay Singh sacrificed his life for the rights of the people of Bihar and to ensure that they get justice. The state showed barbarity by ordering a lathicharge and authorising the use of tear gas leading to his death. The government should resign."
"BJP Members of Parliament, MLAs and workers have been injured in the hundreds and are admitted in different hospitals," he added.
When it was pointed out to him that the Patna District Magistrate is not confirming this, Mr Rai said he should be asked whether he is looking at the situation through the eyes of Chief Minister Nitish Kumar and his deputy Tejashwi Yadav. "The people will ask these questions of the District Magistrate, Nitish ji and Tejashwi ji," Mr Rai said.
The party's protest, which started from Gandhi Maidan in Patna, was against corruption and the government doing away with the domicile requirement for the recruitment of teachers.
The BJP got more ammunition for its corruption allegations after Tejashwi Yadav was named in the CBI chargesheet in the land-for-jobs scam on July 3. The scam involves people allegedly being given employment in the Indian Railways from 2004 to 2009 - when Lalu Yadav was the railway minister - in return for land parcels being gifted or sold at cheap rates to the Yadav family.
Nitish Kumar and Tejashwi Yadav have been at the forefront of efforts to unite the opposition and had organised the mega 16-party meeting in Patna last month.
The BJP and Mr Kumar have been at loggerheads ever since he snapped ties with the party last year and formed a government with the Rashtriya Janata Dal and the Congress. After the rebellion in the NCP, BJP's Sushil Kumar Modi had said earlier this month that the party will not accept Mr Kumar into its fold at any cost, even if he "rubs his nose" on its door.