Dalerdzhon Mirzoyev, a suspect in the Crocus City Hall shooting on Friday is escorted by a FSB officer in the Basmanny District Court in Moscow, Russia, March 25, 2024. (PHOTO / AP)

MOSCOW - Russians across the country were mourning over the weekend the victims of a terrorist attack in Moscow that claimed at least 137 lives, as four individuals directly involved in the attack have been charged with terrorism.

A Moscow district court on Monday sent all four suspects, identified as citizens of Tajikistan, to pre-trial detention. They will remain in custody until May 22, Russia's TASS news agency reported.

On Friday night, gunmen fired indiscriminately at concertgoers at Crocus City Hall in suburban Moscow, leaving at least 137 people dead. A large blaze later led to the collapse of the building's roof.

Russian President Vladimir Putin said in a televised address on Saturday that a preliminary investigation showed the Ukrainian side was preparing a "window" for the perpetrators to cross the border. Calling the attack "bloody and barbaric", Putin promised to identify and punish all those behind the attack.

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

People lay flowers to mourn victims of a terrorist attack in Moscow, Russia, March 24, 2024. (PHOTO / XINHUA)

He also declared March 24 a day of national mourning. "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."

On Sunday, near the Crocus City Hall, a somber sea of Russian citizens of all ages converged to pay their respects to the victims of the attack.

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.

This photo taken on March 24, 2024 shows flowers laid by people to mourn victims of a terrorist attack in Moscow, Russia. (PHOTO / XINHUA)

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.

This photo taken on March 24, 2024 shows flowers, candles and toys laid by people to mourn victims of a terrorist attack in Moscow, Russia. (PHOTO / XINHUA)

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.

People place flowers and toys at the fence next to the Crocus City Hall, on the western edge of Moscow, Russia, March 24, 2024. (PHOTO / AP)

"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."