Rajgriha (modern-day Rajgir in Bihar) was the early capital of the ancient kingdom of Magadh. It was the seat of power during the rule of the Haryanka dynasty, especially under King Bimbisara and King Ajatashatru. Later, the capital was shifted to Pataliputra (modern-day Patna). Answer: $\boxed{\text{Rajgriha}}$