Kremlin critic Alexei Navalny, who fell gravely ill after what his allies believe was a poisoning, was under intense police surveillance in preceding days, a Russian tabloid newspaper has cited law enforcement sources as saying.

Before he collapsed on Thursday on a flight during a trip to Siberia, Mr Navalny was followed by plain-clothed FSB officers and his movements were closely monitored via CCTV, the report in the Moskovsky Komsomolets newspaper said.

Mr Navalny, a long-time opponent of President Vladimir Putin and a campaigner against corruption, was flown in an air ambulance on Saturday for treatment in Germany.

Mr Navalny, 44, was in an induced coma when he was evacuated from the Siberian city of Omsk, but there has been no word yet from the Charite hospital in Berlin on his condition.

German army emergency personnel load a portable isolation unit into their ambulance that was used to transport Russian opposition figure Alexei Navalny. Photo: Getty

Citing security service sources, Moskovsky Komsomolets described the timeline of his trip before he fell ill down to the number of rooms his team booked in a hotel and the fact that Mr Navalny chose not to sleep in the room booked under his name.

An apartment rented for him by one of his supporters was discovered by police surveillance, the paper reported, when a sushi takeaway was ordered to the address by one of Navalny’s supporters.

“The scale of the surveillance does not surprise me at all, we were perfectly aware of it before,” Mr Navalny’s spokeswoman Kira Yarmysh wrote on Twitter.

“What is surprising, however, is that (security service sources) did not shy away from describing it.”

In its report, the Moskovsky Komsomolets paper cited security sources as saying that their surveillance of Mr Navalny’s movements did not reveal any suspicious contacts that could be related to his illness.

Russian dissident politician Alexi Navalny is believed to have been poisoned. Photo: Getty

Security services believe that if Mr Navalny was poisoned, the incident took place either in the airport or on the plane, the newspaper wrote.

However the paper said they are still awaiting results of laboratory tests of samples taken by police from all the places Mr Navalny and his team visited on their trip, including samples of the air.

Initial results are expected on Monday, with results from tests for radioactive material due later in the week, the paper said.

It did not say whether or not these would be made public.

Doctors at the hospital in Omsk where Mr Navalny was treated before his evacuation to Germany have said they do not believe he was poisoned.

They diagnosed him with a metabolic disease that may have been caused by low blood sugar.

Kremlin spokesman Dmitry Peskov said on Friday that it was still unclear what caused Mr Navalny to fall ill.

He had previously said that any poisoning would need to be confirmed by laboratory tests and that doctors were doing everything they could to help Mr Navalny.