Rahul Mahajan is still in hospital

The arrested man, Rahul Mahajan, was last week admitted to a Delhi hospital. Police said they had "sufficient evidence" to arrest him.

One of his father's aides, Vivek Moitra died during the evening.

Mr Mahajan's father, Pramod, was shot dead last month, apparently by his own brother in a domestic incident.

The incident happened at a time when correspondents say he was poised to play a leading role in reviving the fortunes of the ailing BJP Hindu nationalist party.

Results of a post mortem on Mr Moitra are still awaited.