People lay flowers to mourn victims of a terrorist attack in Moscow, Russia, March 24, 2024. [Photo/Xinhua]

Near the Crocus City Hall in suburban Moscow, a somber sea of Russian citizens of all ages converged to pay their respects to the victims of Friday's terrorist attack here that claimed at least 137 lives.

Amid the bleak grey day, an impromptu tribute featuring predominantly red and white flowers left by mourners expands by the minute. Among the candles and blooms, fluffy toys contributed by children are scattered. Periodically, sirens can be heard in the background as search work at the site continues.

In a televised address to the nation on Saturday, Russian President Vladimir Putin declared March 24 a day of national mourning for the lives lost in the terrorist attack. More than 150 people were injured after the attack, with some still in critical condition.

"I express my deep, sincere condolences to all those who lost their loved ones," Putin said in his address. "The whole country and our entire people are grieving with you."

Near the city hall, approximately 200 meters to the east, traffic moved slowly on Sunday as congestion built up, caused by numerous vehicles belonging to those arriving to offer their condolences.

Marina, a Moscow resident at the mourning site near the concert hall, described the attack as "a suffocating tragedy." She was still in disbelief that "Those people's lives were just taken away."

Vladimir, another resident at the site, said the victims were ordinary people, some of them children. "Those who carried out the attack deserve the severest punishment," he added.

In central Moscow, landmarks such as the Kremlin and the State Duma building flew their flags at half-mast on Sunday. In the tourist hotspot New Arbat Street, an electronic display spanning tens of meters reads: "Russia always becomes stronger in the face of the toughest challenges, and it will be so now."

Since Friday night, billboards throughout Moscow and across the country have displayed candle patterns in memory of the victims of the terrorist attack.

In Saint Petersburg, Russia's second-largest city, citizens gathered spontaneously at multiple locations to lay flowers, light candles and observe moments of silence for the victims.

"I hope those who ordered this terrorist attack are brought to justice in some way," said Stanislav, a Saint Petersburg citizen. "Everyone must understand that terrorism is an unacceptable crime."

In his Saturday speech, Putin called the terrorist attack bloody and barbaric, vowing retribution. All four gunmen involved were detained, Putin said, adding that all perpetrators, organizers and financiers of the crime will be punished.

"No one and nothing can shake our unity and will, our determination and courage, the strength of the united people of Russia," Putin said.