"PM Modi is for rule by stealth, deception and pressure. He wants to keep everyone on tenterhooks so the officers do the government's bidding. He wants to keep a sword hanging over their heads," Congress spokesperson Abhishek Manu Singhvi said.
The remarks came after Congress leader in the Lok Sabha Mallikarjun Kharge wrote a strongly worded letter to PM Modi against Asthana's appointment.
Lawyer and activist Prashant Bhushan have already taken the matter to the Supreme Court.
Terming the appointment, "constitutionally, legally and morally wrong," Singhvi said, "rules were thrown to the wind."
Kharge who is the part of the panel which selects CBI director in his capacity as leader of the largest opposition party in the Lok Sabha, urged the PM in his letter dated December 5 to convene the meeting of the committee at the earliest to finalise a permanent CBI chief.
Kharge charged the government did not call the meeting of the selection committee headed by the PM deliberately to facilitate the appointment of a junior officer to the post.
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''The entire process has been vitiated and being manipulated to preempt the decision to be arrived at in the meeting of the selection committee," Kharge said in his letter to the PM.
Besides the PM and Kharge, Chief Justice of India TS Thakur is a member of the panel.
Asthana is a 1984 batch Indian Police Service officer and has replaced Anil Sinha who retired as CBI director on November 30.
Kharge pointed out that the selection panel should have met in time to avoid any delay in the matter.
He also referred to the sudden transfer of RK Dutta, who was in the line of succession to be the next CBI chief just three days before Sinha retired.
Sources said Dutta, who has been moved to the home ministry as special secretary, was looking after two high-profile corruption cases related to allocation of coal mines and 2G Spectrum during the previous UPA.
The sources pointed out that the Supreme Court had ordered that no officers connected with the two cases should be removed.
"All this has been done to make the CBI even a more pliant tool in the hands of the government," said Singhvi.
According to the Congress spokesperson, also a senior Supreme Court lawyer, "similar tactics were adopted by PM Modi earlier in the appointment of the ED director."
In October 2015, the centre had appointed Girish Chandra Murmu as the Director of Enforcement Directorate. A 1985-batch IAS officer, Murmu had replaced 1984-batch IPS officer Karnal Singh who was holding the additional charge as ED chief after 1979-batch IAS officer Ranjan Katoch was removed from the post in August.
In August, 2015, BJP leader Subramanian Swamy had urged PM Modi to appoint a full-time ED director alleging that the agency had given a clean chit to Congress chief Sonia Gandhi and Vice president Rahul Gandhi in a money laundering case related to now defunct newspaper National Herald.