Specks of black oil have rained down on part of Moscow after a refinery was hit during the largest Ukrainian attack since the start of the full-scale war, with close to 200 drones fired towards the Russian capital.
Columns of thick smoke billowed high into the sky and 17 people were wounded in the Moscow region, according to local governor Andrei Vorobyov.
Residents in the south-east of Moscow region told the BBC that a fine drizzle had left "unpleasant black spots" on their clothes.
Moscow authorities denied that any "oil rain" had been falling.
However, the city's official Telegram channel warned residents of the affected district to keep their windows closed and said families with children, elderly people and asthmatics should urgently leave the area.
Almost 1,000 drones and four Ukrainian cruise missiles were intercepted and destroyed across the country in 24 hours, Russia's defence ministry was quoted as saying. An oil depot was struck in the southern Rostov region, where one person was killed.
Ukraine's Volodymyr Zelensky said the drone strike was an answer to last week's Russian attack on Kyiv, which set ablaze a major religious landmark, the Pechersk Lavra monastery.
"We don't want this war and have never wanted it," Zelensky said. "But if Ukraine burns, your Moscow will burn too."
In response, Russian Foreign Minister Sergei Lavrov said strikes on Ukraine would be delivered "on a mass scale", adding he had been "convinced for a long time that words are not enough".
Fires broke out as the Kapotnya refinery in south-east Moscow was hit for the third time in a month and the second time this week, colouring the sky black with smoke. Several clips show the particularly dramatic moment the top of a large silo was blown off by a huge explosion, sending the roof of the oil storage tank flying dozens of metres into the air.
A nearby shopping centre also caught fire, reportedly after drone debris fell on the building. In a video verified by the BBC, a drone can be seen crashing into the upper floors of a high-rise building, with glass and debris raining down its facade and into the courtyard beneath.
In another verified video, a thick, dark, oily sheen could be seen coating the tarmac of a car park, while the ground beneath parked vehicles remains clear.
"As soon as I stepped out of my apartment building, there was this fine, light drizzle," one local woman told the BBC.
She noticed "unpleasant black spots" on her clothes and her friend's jacket, too, "ended up covered in black specks," she added. "We'll now be keeping an eye on whether our hair starts falling out because of petroleum products."
Moscow's four airports were temporarily shut and more than 500 flights were cancelled or delayed.
Although local authorities across Russia have banned publication of images of the aftermath of drone strikes, dozens of videos were posted on social media showing drones flying across the sky in broad daylight and explosions over industrial areas on the outskirts of Moscow.
It has been a regular Ukrainian tactic to launch a large number of reconnaissance decoy drones to map out the density of air defences and vulnerable areas, before the main air strikes begin.
Four and a half years since Moscow launched its full-scale invasion of Ukraine, the war of attrition on the front line in Ukraine grinds on, out of sight for many in Russia. Kyiv's long-range strikes on targets across Russia as well as Moscow and St Petersburg are an indication of Zelensky's aim of "bringing the war home" to ordinary Russians.
Drone attacks on Moscow - about 500km (310 miles) from the Ukrainian border - have become more frequent as Kyiv has developed its long-range capabilities. Ukraine's first successful drone strikes reached the Russian capital in spring 2023, although they were sporadic and rarely involved more than a handful of drones.
Since then, extensive air defences have been set up around Moscow - but the number of drones used by Ukraine in its attacks has also multiplied and some have penetrated those defences.
No air defence system can ensure total protection against massive attacks of high-tech drones. The hit rate of those that do get through is extremely low and fraught with the risk of anti-missile debris crashing to the ground.
But despite the known difficulties in halting such large-scale attacks, Thursday's drone barrage is bound to raise questions about the effectiveness of air defence systems surrounding key infrastructure in Moscow.
For its part, Russia launched more than 200 drones and multiple ballistic missiles at Ukraine overnight, Kyiv said.
Vladimir Putin, who is hosting southeast Asian leaders for a summit in the central city of Kazan, has not commented on the large-scale attack on the Russian capital.
Writing on X, Ukrainian Foreign Minister Andriy Sybiha said: "One of the most popular questions asked by Muscovites this morning is 'What is going on?'"
"I can answer. Your country started a war of aggression against ours. For years, it has been killing our people," Sybiha wrote.
"Now that you know what's going on, ask Putin when he is planning to end it."
Additional reporting by Ilya Abishev, BBC Russian and BBC Verify's Richard Irvine-Brown and Paul Brown
