Russia accused Ukraine on Wednesday of attacking the Kremlin with drones overnight in a failed attempt to kill President Vladimir Putin.
A senior Ukrainian presidential official denied the accusation - the most serious that Moscow has levelled at Kyiv in more than 14 months of war - and said it indicated Moscow was preparing a major "terrorist provocation".
The Kremlin said Russia reserved the right to retaliate, and hardliners demanded swift retribution against Ukrainian President Volodymyr Zelenskiy.
"Two unmanned aerial vehicles were aimed at the Kremlin. As a result of timely actions taken by the military and special services with the use of radar warfare systems, the devices were put out of action," the Kremlin said in a statement.
"We regard these actions as a planned terrorist act and an attempt on the president's life, carried out on the eve of Victory Day, the May 9 Parade, at which the presence of foreign guests is also planned ...
"The Russian side reserves the right to take retaliatory measures where and when it sees fit."
Baza, a Telegram channel with links to Russia's law enforcement agencies, posted a video showing a flying object approaching the dome of the Kremlin Senate building overlooking Red Square - site of the Victory Day parade - and exploding in an intense burst of light just before reaching it. Reuters could not immediately verify the video's authenticity.
Ukrainian presidential adviser Mykhailo Podolyak said in comments sent to Reuters: "Ukraine has nothing to do with drone attacks on the Kremlin. We do not attack the Kremlin because, first of all, it does not resolve any military tasks."
He added: "In my opinion, it is absolutely obvious that both 'reports about an attack on the Kremlin' and simultaneously the supposed detention of Ukrainian saboteurs in Crimea ... clearly indicate the preparation of a large-scale terrorist provocation by Russia in the coming days."
The powerful speaker of the lower house of Russia's parliament, Vyacheslav Volodin, issued a statement demanding the use of "weapons capable of stopping and destroying the Kyiv terrorist regime".
Margarita Simonyan, head of the state broadcaster RT, wrote on Telegram: "Maybe now things will get started for real?"
PUTIN WAS NOT IN KREMLIN - RIA
The statement from the presidential administration said fragments of the drones had been scattered on the territory of the Kremlin complex but there were no casualties or material damage.
RIA said Putin had not been in the Kremlin at the time, and was working on Wednesday at his Novo Ogaryovo residence outside Moscow.
Another video circulating on Russian social media appeared to show a plume of smoke over the Kremlin after the purported attack.
The video was posted in the early hours of Wednesday on a group for residents of a neighbourhood that faces the Kremlin across the Mosvka River. It was picked up by Russian media, including the Telegram channel of the military news outlet Zvezda.
Victory Day is a major public holiday commemorating the defeat of Nazi Germany in World War Two, and a chance for Putin to rally Russians behind what he calls his "special military operation" in Ukraine.
Russia marks the occasion with a huge military parade on Red Square, for which seating has already been erected.
The state news agency TASS said the parade - for which the Kremlin last week announced tighter security - would still go ahead.
Moscow Mayor Sergei Sobyanin said earlier on Wednesday that the city had introduced an immediate ban on unauthorised drone flights.
Russia has accused Ukraine of numerous cross-border attacks since the start of the war, including strikes in December on an air base deep inside Russian territory that houses strategic bomber planes equipped to carry nuclear weapons. In February, a drone crashed in Kolomna, about 110 km (70 miles) from the centre of Moscow.
Ukraine typically declines to claim responsibility for attacks on Russia or Russian-annexed Crimea, though Kyiv officials have frequently celebrated such attacks with cryptic or mocking remarks.
In late March, however, the head of the Ukrainian National Security and Defense Council, Aleksey Danilov, claimed on Twitter that Ukraine possesses “dozens of models” and “thousands of UAVs,” including those with a range of “more than 3,000 kilometers.”