Cowering in the labyrinth of Soviet-era bunkers far beneath the vast Azovstal steelworks, Natalia Usmanova felt her heart would stop as Russian bombs rained down on Mariupol, sprinkling her with concrete dust.

Usmanova, 37, spoke on Sunday after being evacuated from the plant, a sprawling complex founded under Joseph Stalin and designed with a subterranean network of bunkers and tunnels to withstand attack.

“I feared that the bunker would not withstand it – I had terrible fear,” Usmanova said, describing the time sheltering underground.

“When the bunker started to shake, I was hysterical, my husband can vouch for that. I was so worried the bunker would cave in.”

She recalled the lack of oxygen in the shelters and the fear that had gripped the lives of people hunkered down there.

“We didn’t see the sun for so long,” she said, speaking in the village of Bezimenne in an area of Donetsk under the control of Russia-backed separatists about 30km east of Mariupol.

Usmanova was among dozens of civilians evacuated from the plant in Mariupol, a southern port city that has been besieged by Russian forces for weeks and left a wasteland.

She said she joked with her husband on the bus ride out, in a convoy agreed by the UN and the International Committee of the Red Cross, that they would no longer have to go to the lavatory with a torch.

“You just can’t imagine what we have been through – the terror,” Usmanova said. “I lived there, worked there all my life, but what we saw there was just terrible.”

Ukraine president Volodymyr Zelenskiy said about 100 civilians, primarily women and children, were expected to arrive in the Ukrainian-controlled city of Zaporizhzhia from the plant on Monday.

“For the first time in all the days of the war, this vitally needed (humanitarian) corridor has started working,” he said in an address published on Telegram. He said he hoped the evacuations would continue on Monday.

People fleeing Russian-occupied areas in the past have described their vehicles being fired on, and Ukrainian officials have repeatedly accused Russian forces of shelling evacuation routes on which the two sides had agreed.

As many as 100,000 people may still be in blockaded Mariupol, including up to 1,000 civilians hunkered down with an estimated 2,000 Ukrainian fighters beneath the Soviet-era steel plant – the only part of the city not occupied by the Russians.

Mariupol has been a key target for Vladimir Putin because of its strategic location near the Crimea Peninsula, which Russia seized from Ukraine in 2014.

Reuters and the Associated Press contributed to this report