Sahai, a 1986-batch Union Territory cadre officer, took charge from D M Spolia, who retired today, official sources said.
A regular appointment for the top bureaucratic post of the Delhi government will be made soon, they said.
Earlier, Home Ministry officials said senior IAS officer R S Negi, who was the first choice of Delhi Chief Minister Arvind Kejriwal for the Chief Secretary's post, is unlikely to be appointed for the post as he is too junior and not in the apex pay scale.
As per the first option, the Home Ministry has sent a panel of three senior IAS officers to the Delhi government to pick up any of them. Negi's name is not in the list.
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As per the second option, the Home Ministry has asked the city government to send name of any officer who is in the pay scale of Rs 80,000.
The Sources said the Delhi government is likely to accept the second option and recommend name of an officer who is in the apex pay scale for appointment of national capital's next Chief Secretary.
Kejriwal on Thursday had requested Home Minister Rajnath Singh to do the needful for appointing 1984-batch IAS officer Negi as the Chief Secretary of Delhi to succeed Spolia.
The Home Minister was said to be ready to accommodate the request of Kejriwal but service rules came in the way.
Negi is currently serving as the Chief Secretary of Arunachal Pradesh.