While Mr Navalny was behind bars, police rounded up more than 1,000 people in the Russian capital at the rally on Saturday (local time) in one of the biggest crackdowns in recent years against the opposition.
Navalny was jailed for 30 days last week after calling for unauthorized protests in Moscow on July 27 that ended with more than 1,300 people detained, according to an independent group that monitors crackdowns on demonstrations in Russian Federation.
Navalny's spokeswoman, Kira Yarmysh, wrote on Twitter that he was transported from a detention facility in Moscow to a hospital due to an "acute allergic reaction" that caused his face to swell and parts of his skin to turn red.
Vasilyeva says she and Ashikhmin were nevertheless able to examine Navalny through a door that had been left cracked open.
Access to Navalny is restricted, and Dr. Anastasiya Vasilyeva, who has been Navalny's physician for several years, only managed to see him Sunday afternoon. Vasilyeva said that she would be allowed to visit Navalny again later on Monday.
The personal doctor of Alexei Navalny has said the jailed Russian opposition leader who was rushed to hospital on Sunday might have been exposed to an unidentified "toxic agent", while health officials insisted his condition was satisfactory. Kazakhmedov said that the medics couldn't immediately say what caused it.
Leonid Volkov, an ally of Navalny, said he had a similar allergic reaction after serving time last month in the same jail. After being called in, police told them to disperse since they were disturbing medics and patients. They're lying to us.
Allies say he risks another flare-up if traces of the substance they fear poisoned him are still present in his cell.
Several would-be candidates were detained before or during the protest on Saturday.
Police put participation at more than 3500 people, of whom it said around 700 people were journalists and bloggers.
As of Monday afternoon, more than 8,500 people had signed up to join the protest on Facebook. According to OVD-Info, an independent site that monitors arrests, 1,373 people were detained in connection with Saturday's unsanctioned protests, which came a week after a legal demonstration in support of free and fair elections drew over 20,000 people, according to White Counter, a group that monitors attendance figures at protests.
Several protesters were dragged into police cars.
She said she found the fact that she had not been allowed to examine him properly to be suspicious.
Opposition leaders have said they will call another protest for this coming Saturday, 3 August, in an attempt to keep up the pressure on authorities.
The rally comes amid wider public frustration over declining living standards that has hit President Vladimir Putin's approval ratings. State TV has also largely ignored the unrest, but millions of Russians watched live streams of Saturday's events on YouTube.