Singh read and signed an oath on Saturday in the presence of Kalam, the ceremonial head of state. Members of the incoming cabinet followed Singh by taking their oaths. Their portfolios were to be announced later.
Other cabinet ministers who were among the first to be sworn in included Congress veteran Arjun Singh, former defence minister Sharad Pawar and former Bihar state chief minister Laloo Prasad Yadav.
Outgoing prime minister Atal Behari Vajpayee, whose Bharatiya Janata party-led alliance was defeated in the election, smiled and shook Singh's hand before the handover of power.
Singh, after signing his oath, folded his hands in a traditional
greeting to Kalam and then to a beaming Sonia Gandhi, the
Italian-born chief of the Congress party who declined to become prime minister.