Unverified video and images on social media showed a huge ball of flame blasting into the night sky and multiple detonations thundering across a lake about 380km west of Moscow.
NASA satellites picked up intense heat sources emanating from an area of about 14 square kilometres at the site in the early hours and earthquake monitoring stations noted what sensors thought was a small earthquake in the area.
"The enemy hit an ammunition depot in the area of Toropets," said Yuri Podolyaka, a Ukrainian-born pro-Russian military blogger.
"Everything that can burn is already burning there (and exploding)."
Ukrainian President Volodymyr Zelenskiy, speaking in his nightly video address, hailed the outcome of the attack without referring specifically to the target.
"A very important result was achieved last night on Russian territory and such actions weaken the enemy," Zelenskiy said.
"I thank everyone involved. Such precision is truly inspiring."
He thanked the SBU security service, the HUR intelligence service and the Special Operations Forces.
Our brigades are holding the front line thanks to the bravery of Ukrainians and the support of our partners. Right now, it is especially important to feel the impact of our agreements at the front lines.— Volodymyr Zelenskyy / Володимир Зеленський (@ZelenskyyUa) These agreements must be implemented as quickly as possible—the course of… pic.twitter.com/Aa8yuL3IJDSeptember 18, 2024
A source in Ukraine's SBU state security service had earlier told Reuters the drone attack had destroyed a warehouse storing missiles, guided bombs and artillery ammunition.
Russian state media have in the past reported that a major arsenal for conventional weapons was located at the site of the blasts.
State media, now subject to military censorship laws, was muted in its reporting on Wednesday.
Igor Rudenya, governor of the Tver region, said that Ukrainian drones had been shot down, that a fire had broken out and that some residents were being moved away.
He did not say what was burning.
One woman told Reuters that members of her family had been relocated from Toropets.
"A fire started with explosions," said the woman, who identified herself only as Irina.
Rudenya later said the situation in Toropets was stable as of midday local time and that residents could return.
The fire had been put out and there were no recorded fatalities, he said.
Russia and Ukraine each reported dozens of enemy drone attacks on their territory overnight, with Russian forces advancing in eastern Ukraine.
The size of the main blast shown in the unverified social media video was consistent with 200-240 tonnes of high explosives detonating, George William Herbert of the Middlebury Institute of International Studies at Monterey in California said.
A Toropets chatroom on the Russian social media site VK was flooded with messages of support from other parts of the country and offers of help to people fleeing the town.
Some people were asking whether buildings at specific addresses were still standing.
Some war bloggers asked how drones could trigger such large blasts at what was thought to be a highly fortified facility.
According to an RIA state news agency report from 2018, Russia was building an arsenal for the storage of missiles, ammunition and explosives in Toropets, a 1000-year-old town, with a population of about 11,000.
