Sengar, the MLA from Bangarmau in Unnao district, was arrested after being questioned at the CBI office here.
The agency filed three FIRs in the case late on Thursday over the rape and the victim's father's death in police custody.
The move came following a request to the Centre from the Uttar Pradesh government.
In the first FIR, the CBI named Sengar and Shashi Singh, a woman who allegedly took the rape victim to the Bharatiya Janata Party leader's house.
The second FIR is against four of Sengar's accomplices -- Vineet, Baua, Shailu and Sonu -- on charges of rioting, criminal intimidation and voluntarily causing hurt.
The third one is a cross FIR filed by Sengar's brother Atul Singh, who claimed that he was attacked by the victim's father. Atul Singh is currently in judicial custody.
The CBI is planning to take over the case papers from the Special Investigation Team (SIT) which includes the statements of the victim, witnesses and family members.
Sengar was asked to join the investigation when he was about to leave for Varanasi. He was brought here early on Friday.
The victim tried to set herself ablaze outside Chief Minister Yogi Adityanath's residence on Sunday, claiming inaction in the rape case that took place in 2017.
Later on Sunday, her father was picked up by the police and allegedly roughed up by Atul Singh and his aides, following which he died.
The incident has triggered outrage.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)