Navalny, 44, a prominent opponent of Russian President Vladimir Putin, started refusing food on March 31 in protest at what he said was the refusal of prison authorities to treat him properly for acute back and leg pain.
"His condition is indeed critical," said Alexandra Zakharova, a representative of the Doctors Alliance trade union - a group that Russian authorities regard as opposition activists.
Navalny, one of President Vladimir Putin's most prominent critics, has separately complained of acute back and leg pain and accused authorities of refusing him access to his chosen doctor and of declining to supply him with the right medicine.
OVD-Info, which monitors the detention of political protesters and activists, published a list of more than 150 people it said had been detained.
"The police came to the forum of municipal deputies in Moscow. There are 150 people here from all over the country. Everyone is being detained. I mean, everyone," opposition politician Ilya Yashin wrote on Twitter.