Hospital officials downplayed the seriousness of the incident, but underlined it was better to take all necessary precautions.
"Girgory Alexeyevich has been coming in for checkups a lot recently... this doesn't always mean that the situation is urgent, but this time we think his condition is moderately serious, and with this kind of diagnosis it's better not to joke," a hospital official said.
Yavlinsky is currently under constant observation by doctors, who plan to carry out a full diagnosis of his condition.
Doctors said that he has an unstable heart condition that carries a high risk of a heart attack and sudden death.
Yavlinksy has also had to forego an unsanctioned rally to take place Sunday in front the Ostankino television tower. Opposition activists plan to protest a documentary about the recent mass rallies that aired on state-controlled channel NTV, which they said was inaccurate and lied about some aspects of the rallies.