Anna Hazare team member Manish Sisodia said they have received a letter from Delhi Police giving permission for holding the protest at Jai Prakash Narain Park for two-and-a-half days.
"They have asked us to given an undertaking that we will end our fast at Jai Prakash Narain Park in that period. This is not acceptable to us," he said.
"We are discussing the matter. We may now sit at Jantar Mantar," he said.
Hazare was denied permission by Delhi Police to hold fast at the capital's protest hotspot Jantar Mantar and instead offered Jai Prakash Narain Park as the venue.
Lokpal bill drafting committee member Prashant Bhushan slammed the undertaking as totally unconstitutional.
"They (Delhi Police) are saying you first give an undertaking that you will move out in 2 or 3 days. Then we will given you permission. Requiring such an undertaking is absurd and totally unconstitutional. Therefore, there is no question of giving any such undertaking," Bhushan said.
The government on Friday denounced the agitation as "unjustified" with a message that it could intervene if Hazare's health fails.
Home Minister P Chidambaram said Hazare's "extra Parliamentary protest" at this stage "seems unjustified" but was evasive on the delay in grant of permission, contending that the matter rests with Delhi Police.
Hazare's earlier fast, when the Lokpal bill was not in place, was "perhaps right" but not now when a bill has already been introduced in Parliament and the government has "moved forward" on it, the Home Minister said.
At the same time, Chidambaram said, "Everybody has a right to protest and the context and circumstances will decide whether the protest was right or not." Hazare's fast at Jantar Mantar in April had generated a nationwide agitation, forcing the government to set up a joint committee to draft Lokpal Bill.