Dramatic footage shows the moment the Chelyabinsk Tractor Plant, which is in the Ural mountains close to the border with Kazakhstan, goes up in flames.
Locals reported a powerful 'explosion', and footage taken by passersby shows a fireball rising high into the sky as the factory burns.
The precise cause of the inferno is not yet known, although key Russian installations have repeatedly been hit by sabotage or kamikaze drone attacks.
Chelyabinsk would appear out of range for drones from Ukraine but a criminal probe has been launched into the precise cause of the explosion.
The plant is under US and Ukrainian sanctions as 'an enterprise specialising in the production of diesel engines for military equipment for the needs of the Armed Forces of the Russian Federation'.
It is a key supplier for a range of key military firepower such as the T-72, T-90 Proryv tanks, the BMPT Terminator, Akatsiya self-propelled artillery units, and the Msta-S and Koalitsiya-SV complexes.
The initial Russian explanation was a transformer explosion caused by a 'short circuit'.
However, a short circuit is used to explain any fire at a key installation in Russia and the authorities routinely play down fires at key facilities during the war.
Comment: It is true that the Russian authorities have not publicly commented on the sheer scale of the sabotage operations.
The plant is currently used to repair military equipment for use in Putin's war against Ukraine, it is understood.
Local sources said later that the fire was under control but it was unclear to what extent the blaze had impacted on the plant's work.
The destruction came a day after Ukraine launched a major kamikaze drone attack on Russia in their heaviest onslaught for months.
Comment: To call it an 'onslaught' is reaching.
Three of Moscow's international airports - Vnukovo, Domodedovo and Zhukovsky - were closed with flights suspended or cancelled due to the attack.
The city's mayor Sergei Sobyanin complained of a 'mass attack' on the Russian capital.
The attacks come one day after the heaviest strikes on Kyiv by Putin kamikaze drones since the start of the war 22 months ago - when 71 out of 75 Iranian-made Shahed UAVs were downed by Ukrainian air defences.
Comment: Analysts have speculated that it was a reconnaissance mission intended to identify what Kiev had as protection.
Russia was forced to shoot down 11 drones overnight and another nine early this morning.
Three people, a man and two women, required medical help and people were evacuated from the building.
Later in the morning a new drone swarm was reported in Tula, a region where Putin crony and former bodyguard Alexei Dyumin is governor, and once saved the dictator from being savaged by a brown bear.
'Loud explosions' were reported over the city. 'It flew right over us,' said one local amid unconfirmed reports of two kamikaze drones being shot down.
And a mysterious fire ignited an elite Mosfilmovskaya Street in Moscow, blocking all traffic according to state media reports.
'The circumstances of the incident and information about the victims are being clarified,' reported Astra media outlet which suggested an 'electrical' inferno.
In the Moscow region, surrounding the capital, five drones were reportedly shot down in Naro-Fominsk, Odintsovo, Ramensky and Podolsk urban districts.
In three cases there was damage to buildings on the ground. It was not immediately clear if any of the drones had hit strategic targets.
Comment: They clearly didn't, because the Kiev-junta would surely have let us know.
More drones were downed in Kaluga, Bryansk and Smolensk regions as Ukraine responded to the onslaught on Kyiv a day earlier.
Mayor Sergei Sobyanin said the main target was Moscow. 'A mass drone attack was attempted overnight,' he said.
Comment: Key word: 'attempted'.
Elsewhere power was cut in the Russian-occupied Donetsk People's Republic after an apparent Ukrainian strike on the region's power grid.
Putin-appointed occupation leader Denis Pushilin said: 'At night, the enemy tried to damage the region's energy system.
'Unfortunately, due to the massiveness of the attacks, not everything was shot down.'
Parts of Donetsk city and Makeevka plus almost all of Mariupol were without power.
The Russian attack in Kyiv on Saturday was the most intense of the war.
Serhiy Popko, head of the capital city's administration, called it 'the most massive air attack by drones on Kyiv'.
'Our soldiers shot down most of the drones. Unfortunately, not all,' said Ukrainian president Volodymyr Zelensky.
'But we continue to work to strengthen our air defence and shoot down more.'
Overnight another nine Iranian drones were launched by Russia on Ukraine.
Eight were reportedly shot down.