The footage was recorded at the Dragon Zoo in Makhachkala, and shows the toddler, named Nikita, standing with his back to the beast when the lion lunges forward, seizes the boy by his coat and drags him to the ground with terrifying speed and power.
Incredibly, the video appears to show that the lion is not in a cage or enclosure but simply tethered inside an open shelter by a simple leather strap.
The mother of the boy screams as she realises what is happening, and throws herself at the wild animal. Acting with desperate maternal instinct, she manages to wrest a crying Nikita away from the jaws of the beast.
The heart-stopping drama was recorded by Aleksei Tadarov, another visitor at the ramshackle 'zoo' in the Republic of Dagestan, an autonomous region in the south-west of Russia.
Aleksei was appalled that his innocuous recording of the animals had capturing a life-or-death drama, thanks to the zoo's apparently uber-relaxed attitude to safety. He posted the footage online, where it was quickly shared by shocked viewers.
The city's deputy prosecutor, Mubariz Mustafayev, responded to widespread indignation on social media. Saying that the incident 'could easily have turned into tragedy', he vowed to investigate the zoo and possibly prosecute its owners.
"Fortunately it turned out well and the boy was not hurt at all, but that hasn't stopped the investigation and our enquiries are continuing. Once the report is prepared a decision will be made on what to charge the managers of the establishment with.
"It is not legal for such a dangerous beast just to be tied up with a very thin leather strap and also certainly not allowed that the lion did not have any covering on its face," he said, in what may be the understatement of the century.