Russian police launch initial 'check' into alleged poisoning of opposition leader

The Russian government has rejected calls for a transparent investigation into the incident, stating it was not possible to determine whether Alexei Navalny was poisoned.

A protester demonstrates in support of Russian opposition leader Alexei Navalny in St. Petersburg, Russia. Source: AAP

Russian police have begun a preliminary examination of opposition leader Alexei Navalny's illness after the Kremlin dismissed German doctors' findings that he was apparently poisoned.

Transport police in Siberia announced on Thursday that they had started "a pre-investigation check" into what led to Mr Navalny's hospitalisation in the city of Omsk, to establish "all the circumstances" and decide whether to open a criminal probe. 

The 44-year-old politician and anti-corruption campaigner fell ill on a plane to Moscow from the Siberian city of Tomsk last week.

Russian opposition figure Alexei Navalny. Source: EPA

The plane made an emergency landing and he spent two days in a clinic at Omsk in a coma before being transferred to Berlin's Charite hospital. 

Kremlin spokesman Dmitry Peskov on Wednesday rejected international calls for a transparent investigation into the incident.

He said the decision was down to law enforcement authorities but that it was not possible to say Mr Navalny was poisoned since no toxic substance had been identified.

German medics said on Monday they do not know the exact substance involved while Mr Navalny was apparently poisoned with a substance that inhibits the cholinesterase enzyme, a feature of nerve agents.

Russian doctors said their tests did not find any trace of poison while they treated Mr Navalny with the same antidote that German medics are using.

Mr Navalny's allies blame Russian President Vladimir Putin and say the opposition figure may have been poisoned by a cup of tea he drank at Tomsk airport.

Medics upload Alexei Navalny into a German special medical plane at the airport in Omsk, Russia. Source: AAP

The preliminary check by police announced a week after the incident is a low-key response after Mr Navalny's allies asked for an investigation into an attempted assassination of a public figure. 

Police do such checks to determine whether a crime has been committed.

It comes after police and plain-clothed security service agents questioned doctors at the hospital without making any public statements.

Transport police said they searched locations that Mr Navalny visited and his hotel room, apparently referring to where he stayed in Tomsk.

They also examined security camera footage and confiscated "more than 100 items that could have value as evidence".

Police said they did not find any "strong-acting or narcotic substances."