As Russia’s invasion of Ukraine enters its fourth day, besieged citizens are uniting around shared fears, faith in their democracy and defiance of the autocrat next door.

Russian troops breached Kyiv’s city limits on Friday night, with gunfire and explosions echoing through the city’s center. Civilians were seeking out shelter in underground bunkers or trying to flee the city as Ukraine’s defense forces outperformed expectations, clinging to their democracy at the edge of the free world.

Kyiv Mayor Vitali Klitschko, the former world champion boxer, issued a citywide curfew for civilians in a bid to keep them indoors throughout the fighting — as well as to thwart attempts by suspected Russian saboteurs. 

A deserted street is seen after Russia launched a massive military operation against Ukraine, in Kyiv, Ukraine, on February 26, 2022. In the inset photo: Ukrainian Danill Rusanyuk, who is 17 years old. He's been studying in Texas for the past six months. (REUTERS/Umit Bektas/Inset: Daniil Rusanyuk)

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Daniil Rusanyuk is a 17-year-old Ukrainian foreign exchange student who has been studying in Texas for the past six months. He gave his age as 16 at first — before correcting himself. His birthday was earlier this week, but he’s had a lot on his mind.

"It’s like a birthday present," he said of the invasion. 

"A very bad birthday present."

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On Feb. 25, 2022, in Kyiv, Ukraine, a woman works to clear debris in her apartment after the building was heavily damaged in military operations. (UNICEF)

The rest of his family is in Kyiv, where he was born and raised. He said his cousin texted him from a fallout shelter — where she said a woman gave birth as gunfire and explosions echoed throughout the city. His parents can hear shooting from their home in central Kyiv at night, and two of his younger cousins are hiding out in a bunker. 

Two friends, he also said — each 18 — texted him saying they planned to help defend Kyiv.

"I am very worried about that," he admitted. 

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He also reiterated reports of Russian saboteurs marking buildings and areas around Kyiv for attack. Many of them are believed to have arrived in advance. They're believed to have rented apartments and laid low until the invasion began, he said.

Authorities have been asking civilians to cover up such marks and remain vigilant.

At the same time, people around the world are watching Ukraine’s resolve. The government handed out thousands of automatic rifles to civilians, who have formed militias to defend their homes, people on the ground tell Fox News Digital. 

FILE PHOTO: Ukrainian President Volodymyr Zelenskiy speaks confirming that he is still in Kyiv

Ukrainian President Volodymyr Zelenskyy speaks alongside other Ukrainian officials in the governmental district of Kyiv, confirming that he is still in the capital, in Kyiv, Ukraine, on Feb. 25, 2022 — in this screen grab obtained from a handout video.  (Ukrainian Presidential Press Service/Handout via Reuters)

As women and children attempted to flee, the government asked all men between 18 and 60 to join the war effort — and officials say even members of Ukraine's parliament of taken up arms.

"These people are fighting for their freedom, and they’re doing it because they love their country," Daniel Hoffman, a former CIA senior officer and station chief, told Fox News Digital. "They’re shaming us. They’re like, ‘We want to fight for democracy and liberty and freedom and all the things that are enshrined in your Constitution and your Bill of Rights.’"

American and British intelligence officials have said Ukraine’s defense has outperformed Russian expectations, at least in the early stages of the invasion.

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On Thursday, a Russian navy warship ordered a group of 13 Ukrainian soldiers to lay down their arms and surrender, video shows. "Russian warship," they replied, "go f--- yourself." 

All were killed in the ensuing bombardment and will be recognized as national heroes, Ukrainian President Volodymyr Zelenskyy said. Now their last words appear on digital road signs in highways around Kyiv, images show, including one Rusanyuk said his father sent him.

Another rallying cry, "Welcome to Hell," has been repeated by official government accounts on Twitter and sprayed over road signs in the path of Russian invaders.

"It certainly has brought people together in Ukraine," Rusanyuk said. "So many people helping out others. It is wonderful to see how they are helping the army, bringing them food."

Ukrainian servicemen take positions at the military airbase Vasylkiv in the Kyiv region, Ukraine February 26, 2022.   (REUTERS/Maksim Levin)

But it’s also been a harrowing week. He shared video he said his mother recorded overnight, from her home near a military hospital in Kyiv.

"I have some friends from Russia and it's actually very strange for me and weird, because we speak the same language … and you just go war on each other?" 

In the darkness, a streak of light flashed through the sky. Automatic gunfire and explosions rumbled along like a passive train, just as they had the night earlier.

"They heard fighting there all night," he said. His father is a military veteran, and they have chosen not to evacuate, even amid concerns they may become trapped if the Russians encircle the city. 

Even if they did, he said, the roads are clogged, fuel is scarce and the government has ordered men between 18 and 60 to stay and fight.

People try to flee Russia's military operation against Ukraine outside Mostyska

A screen grab from drone footage shows cars forming a line that stretches some 35 km from the Shehyni border crossing to Poland as people try to flee Russia's military operation against Ukraine, outside Mostyska, Ukraine, on Feb. 26, 2022.   (REUTERS/Natalie Thomas)

Another harrowing detail: Many people have blood relatives on the other side of the border, Rusanyuk said. His grandmother lives in Russian-controlled Crimea, which Vladimir Putin seized in 2014.

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"I have some friends from Russia and it's actually very strange for me and weird, because we speak the same language … and you just go war on each other?" he said. 

"And I think, too, my friends — like, what's happening over there? And they are also scared because we know each other so good, and we go on with the war [against] each other like brother [on] brother."

Russia’s war on Ukraine has prompted protests around the world, including within Russia itself, and stiff sanctions from the West, NATO, the United States, Asian allies and the European Union.

Surveillance footage shows a missile hitting a residential building in Kyiv

Surveillance footage shows a missile hitting a residential building in Kyiv, Ukraine, on Feb. 26, 2022, in this still image taken from a video. (REUTERS)

On Saturday, after days of delay, the U.S. and the EU in a joint statement announced they would block "selected" Russian banks from accessing the Society for Worldwide Interbank Financial Telecommunication, or SWIFT system, the backbone of international banking. (A senior Biden administration official referred to it as the "Gmail for banks" on Saturday.)

Ukrainian President Volodymyr Zelenskyy has been asking for the move since the start of the invasion.

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The U.S. bolstered its troop presence in Europe toward the end of the week, especially in NATO member nations neighboring Ukraine. 

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On 25 February 2022 in Kyiv, Ukraine, a girl looks at the crater left by an explosion in front of an apartment building which was heavily damaged during ongoing military operations. (UNICEF)

The U.S. and Germany have also pledged to supply weapons after Zelenskyy turned down an American offer to be evacuated from Kyiv. "The fight is here," he said, according to the translation. "I need ammunition, not a ride."

Meanwhile, Rusanyuk and his friends have their own translation for NATO: "No Action, Talk Only."

A Ukrainian serviceman opens the door of a deactivated Russian military multiple rocket launcher on the outskirts of Kharkiv, Ukraine, on Friday, Feb. 25, 2022. Russian troops bore down on Ukraine's capital Friday, with gunfire and explosions resonating ever closer to the government quarter, in an invasion of a democratic country that has fueled fears of wider war in Europe and triggered worldwide efforts to make Russia stop.

"They made some sanctions on Russia," the high schooler said. "They don’t help Ukraine … Putin doesn’t care about that."

Ukraine needs troops and weapons, according to Rusanyuk. "Not sanctions — we need some real things," he said.

However, even with a war raging at home, he said he’d rather be in Kyiv helping out than safe in Texas.

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"My family is there, my little cousins are there — they are sleeping in bunkers," he said. "I would like, just somehow, to help them."

Fox News’ Marisa Schultz and The Associated Press contributed to this report.