India's coastguard stopped and detained the ship off the Indian coast on October 12 after discovering the cache of weapons and ammunition, before escorting it to the southern port of Tuticorin.
Police then launched an investigation into the 10 crew and 25 security guards of the Seaman Guard Ohio which is registered in Sierra Leone and belongs to the US-based maritime security firm AdvanFort.
The 35, who include British, Estonian, Ukrainian and Indian nationals, were detained on Friday over the stash of some 35 assault rifles and around 5,600 rounds of ammunition, according to the Press Trust of India (PTI) news agency.
Thirty three of them were taken to a local police station for questioning, while two remained on board, PTI said, citing police sources.
The crew had not maintained a log of the arms and did not have the proper permits to carry them in Indian waters, according to NDTV, also using unnamed sources.
The incident comes after a furore erupted in India last year over the