As Russia unleashed a furious air assault, the hardiness of Ukrainian civilians took over.

KHARKIV, Ukraine — The text message was short and to the point.

"Strategic bombers in the air… Go to the shelter when missiles come," a government adviser texted NBC News. The message had gone out around 7 a.m. local time Friday (midnight ET).

Within minutes, the air raid sirens started to blare, and people out in the streets rushed to shelters amid loud booms as Russian rockets and drones began falling. First, the booms were far apart and distant—over five successive bursts they got closer and closer to Kharkiv, Ukraine's second largest city in the country's northeast. The final one was pretty loud.

Similar scenes were repeated, he said, in the capital Kyiv, the southern city of Odesa, and across the country in identical scenes. It marked Moscow's heaviest bombardment in weeks but did not seem to portend a shift in the campaign that the Kremlin said on Thursday.

"More than 100 rockets of various types" were fired, along with about 100 Iranian-made Shahed drones, the statement said. "And like most previous Russian strikes, this one is just as vile, targeting critical civilian infrastructure," it added.

On its behalf, Ukraine's air force claimed that Russia fielded the use of 11 TU-95 strategic bombers, depending on their type, which could carry up to six cruise missiles.

At least five people were confirmed dead, including a 69-year-old man in the Dnipropetrovsk region and a man in the Zaporizhzhia region, according to local officials. Other three deaths were reported in the Kharkiv, Zhytomyr, and Volyn regions.

Inside a shelter in Kharkiv, on social media, there was the dawning realization that this was some kind of massive assault drone after drone, missile after missile, incoming from all points of the compass, on paths preordained by Russian mission commanders deep in their bunkers, helixing and spiraling to dodge Ukrainian air defenses.

Videos on social media showed real-time sightings of the missiles flying, falling, and exploding. Others showed people flocking to the metro stations to wait it out.

Prime Minister Denys Shmyhal told a later statement that 15 Ukrainian regions received damage from the barrage.

In turn, Russia's Ministry of Defense said its forces targeted crucial energy assets in the region with high-precision weapons in support of the military-industrial complex. Among the energy targets are power substations, a gas compressor, and storage sites for aircraft weapons.

Meanwhile, Poland reported that an "object" had entered its airspace and might have landed on Polish territory; searches have already begun.

After more than two years of war, Ukrainians seemed to be inured to such strikes, and it came two days after citizens in Kharkiv and more than a dozen other cities across the country took to the streets to celebrate Independence Day and mark the 33 years since the country gained its autonomy from the Soviet Union.

Many flooded social media with messages of gratitude, support and thanks for the Ukrainian soldiers on the front lines. Some praised their surprise incursion into Russia's Kursk region. Since the incursion started on Aug. 6, Ukraine has seized a considerable amount of territory, including scores of small towns, and has captured hundreds of Russian soldiers.

As soon as the alert was lifted after an hour or so of an air raid, Kharkiv quickly returned to normal Monday.

The cafes were packed again: some served food with a "gen" label on — meaning it could be prepared on a generator.