Alexei Navalny was in intensive care and on an artificial lung ventilator, spokeswoman Kira Yarmysh said on social media.
A fierce critic of President Vladimir Putin, Navalny started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning, Yarmysh said.
Navalny's plane later made an emergency landing at an airport in Omsk so that he could be rushed to hospital.
Footage posted on social media by a fellow passenger showed a motionless Navalny being stretchered into an ambulance by medics.
However, one of the doctors there said it was not certain that he had been poisoned. Doctor Anatoly Kalinichenko said "natural poisoning" was one of several diagnoses being considered and tests were being carried out.