By Sarah Rainsford
BBC News, Moscow
A group of Russian deputies has announced a hunger strike in protest against a new law on state benefits that has sparked a wave of rallies.
Deputies say they will stay as long as it takes
The Rodina faction's five MPs said they would refuse food and remain inside parliament until the law replacing social benefits with cash is scrapped.
Ministers responsible for the reform, meanwhile, admitted serious mistakes in its implementation at a heated session.
Thousands of pensioners protested again as the lower house - or Duma - met.
The Rodina faction walked out of the Duma session devoted to the reform which replaces benefits - such as free transport and subsidised medicine for pensioners and invalids - with cash compensation.
The faction is usually loyal to the Kremlin - but its leader, Dmitry Rogozin, now says he and his supporters will stay as long as it takes.
The deputies have already had inflatable mattresses delivered to their offices.
Rodina is also demanding the immediate resignation of three architects of the benefits reform - including the Finance Minister Alexei Kudrin.
The faction complains the pro-presidential block that controls parliament refuses to consider its serious concerns about the law, which has brought thousands of pensioners onto the streets across Russia in protest.
Despite acknowledging serious mistakes in the implementation of the reform, ministers insisted the law itself was sound.
They argued that it was crucial to overhaul an inefficient system.
But thousands of pensioners on Friday's demonstration said the new rules left them far worse off.
The government is now scrabbling for solutions to appease them - but this is already the most controversial and politically damaging episode in Vladimir Putin's presidency by far.