A government statement says doctors diagnosed Anatoliy Popov with "poisoning from an unknown toxin.

"He is currently in one of Chechnya's hospitals and will soon be taken to Moscow. He is in serious condition," the statement said.

Popov has served as acting president during the campaign for the October 5 presidential polls, in which Kadyrov is heavy favorite after his main rivals in the contest either withdrew or were disqualified.

"The diagnosis is poisoning by a substance of unknown origin," the Chechen government's press service said of Popov, who is not running in the election.

"He is currently in one of Chechnya's hospitals and will soon be taken to Moscow. He is in serious condition."

The Chechnya election is a key plank in President Vladimir Putin's plan to end a decade of separatist violence.

Armed rebels, who subject tens of thousands of Russian forces to daily attacks, reject the poll as meaningless and have pledged to disrupt it.