"This is the first arrests in the case of Mumbai blasts and this is a major breakthrough in the investigation a week after the blasts," the official said.

Two of the men were arrested in the northern state of Bihar on Thursday evening while the third man was arrested in Mumbai on Friday morning, Indo-Asian News Service quoted the official as saying, adding that half kg of explosive was found in the possession of one of them.

Hundreds of people have been detained across the country for questioning since the bombings occurred on July 11, but the three men were the first to be formally arrested, the News Service said.

Police were interrogating them over their role in the bombings that killed 207 people and wounded more than 800, said the News Service.