"We will challenge the Centre's decision (to divide AP) in the Supreme Court," Jagan said at the 'deeksha' camp.
Noting that there were "legal problems" involved in the state's division, he wondered how the Centre could find solutions in six weeks.
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"We are opposing the unilateral manner in which the Centre is functioning. How can it go ahead with the bifurcation process without a resolution in the state Assembly?" Jagan asked.
"We have never heard of an instance of state bifurcation without a resolution in the Assembly," he said.
The Kadapa MP asked why the Centre shouldn't change its decision "when there is so much happening in the state", when it could revoke the ordinance (aimed at protecting convicted lawmakers) on the intervention of Congress vice president Rahul Gandhi.
This is the second time that Jagan is undertaking an indefinite fast on the issue.
Last month, he went on an indefinite hunger-strike in the Chanchalguda Jail, where he was lodged as an undertrial in an alleged disproportionate assets case against him, but it was foiled on the fifth day.
Following the Union Cabinet's approval of division of the state, Jagan, after being granted bail in the case, re-launched his agitation blaming the Centre and the Congress for going ahead with the bifurcation process without addressing the concerns of "majority of people".
Scores of YSRC activists gathered at his Lotus Pond residence to greet Jagan as he began the stir at 11.30 AM after paying homage at the bust of his late father Y S Rajasekhara Reddy.
Party MP Mekapati Rajamohan Reddy and other leaders flanked Jagan on the specially erected 'deeksha' platform.