Mr. Kara-Murza suddenly fell ill on Tuesday and was taken by ambulance to the First City Hospital, where he was reported on Wednesday to be in serious but stable condition. Ms. Kara-Murza said that while doctors said they had not found evidence of poisoning, they also had not ruled it out. They also said he was too weak to be transported out of Russia. “Since the doctors cannot come up with a definite source of his intoxication, we do not know how safe he is there,” Ms. Kara-Murza said. “That’s why we are trying to attract as much attention to his condition as we can.”

There is basis for the concern.

Critics of the Kremlin have occasionally died under mysterious and sinister circumstances. A former K.G.B. officer and caustic critic of Mr. Putin, Alexander V. Litvinenko, died of radioactive poisoning in Britain in 2006. The cause of his death was determined to be the rare isotope polonium 210.

Another Russian citizen, Alexander Perepilichny, 44, died in England under mysterious circumstances in November 2012, after helping to expose a huge tax fraud in Russia allegedly involving senior officials.

Aleksei Svet, the chief doctor at the First Moscow City Hospital, told the Interfax news agency that Mr. Kara-Murza appeared to be suffering from double pneumonia and pancreatitis.

Several acquaintances said they had seen Mr. Kara-Murza in recent days and that he had been in good health. Andrey Bystrov, who works with Mr. Kara-Murza at Open Russia, said he saw him Monday and he was fine.

Mr. Bystrov said he was trying not to jump to conclusions, but that it was difficult not to suspect an attack. “We are now in such a phase with Russia, you never know what really can happen,” he said in a phone interview.