Liverpool City Council said on Friday that it would carry out urgent works after inspectors found the Lime Street premises had "deteriorated".
The Futurist Cinema, opened in 1912, was already due to be demolished due to its "poor structural condition" as part of a Â£35m regeneration of the area.
The SAVE Britain's Heritage group said plans should instead focus on repairs.
The organisation's president Marcus Binney said: "While it is right to take precautionary measures if necessary, this should be to hold the building up, not demolish it."
The works follow a safety inspection that found "internal collapse, leaning walls and a high risk that cladding tiles may fall off", a council spokesman said.
On Friday, the council said it was given a legal notice to make the structure "completely safe" within five days.
Mark Kitts, the council's assistant director of regeneration, said they would "make the area around it safe before starting to deconstruct the parts of the structure that are unsafe".
He added that "when we bought the site a couple of years ago it was already pretty much beyond repair".
A spokesman for SAVE said it recognised "the need to improve the area, but strongly condemns these plans to demolish rather than repair and the deliberate neglect over many years of publicly-owned heritage assets that should have been maintained under full repairing leases".
The group is seeking a judicial review of the case at the Court of Appeal.
The spokesman added: "No development works on the site in connection with the planning permission are permitted until the Court of Appeal case has been heard."
The local Save the Futurist campaign said last year that it was "a tragedy" that the structure had been allowed to deteriorate but acknowledged that "years of neglect have led to a situation where the building cannot be salvaged".
Heritage campaigners have said they are "extremely concerned" over emergency work at a derelict former cinema.