Moscow’s Basmanny District Court formally charged Dalerdzhon Mirzoyev, 32; Saidakrami Rachabalizoda, 30; Mukhammadsobir Faizov, 19; and Shamsidin Fariduni, 25, with committing a group terrorist attack resulting in the death of others.

The offence carries a maximum sentence of life imprisonment.

The court on Sunday ordered the men, all of whom are citizens of Tajikistan, be held in pre-trial custody until May 22.

Mirzoyev, Rachabalizoda and Shamsidin Fariduni admitted guilt after being charged.

The fourth, Faizov, was brought to court directly from a hospital in a wheelchair and sat with his eyes closed throughout the proceedings.

The other three suspects appeared in court heavily bruised with swollen faces amid reports in Russian media that they were tortured during interrogation by the security services.

The hearing came as Russia observed a national day of mourning following the attack on Friday on the suburban Crocus City Hall concert venue that killed at least 137 people.

The attack, which has been claimed by an affiliate of the Islamic State group, is the deadliest on Russian soil since the 2004 Beslan school siege, which left more than 300 people dead, more than half of them children.

Russian authorities arrested the four suspected attackers Saturday, with seven more people detained on suspicion of involvement in the attack, Russian President Vladimir Putin said in an address on Saturday.

He claimed they were captured while fleeing to Ukraine, something that Kyiv firmly denies.

Video showed a sombre-looking Putin lighting a candle at a church at his residence outside Moscow on Sunday evening to honour those who died.

Cultural events were cancelled, flags were lowered and TV entertainment and ads were suspended.

A steady stream of people took flowers to a makeshift memorial near the Crocus City Hall music venue.

As rescuers continue to search the damaged building, some families still do not know if members who went to the event targeted by gunmen on Friday are alive.

Igor Pogadaev was desperately seeking any details of his wife’s whereabouts after she went to the concert and stopped responding to his messages.

He has not seen a message from Yana Pogadaeva since she sent her husband two photos from the venue.

After Pogadaev saw the reports of gunmen opening fire on concertgoers, he rushed to the site but could not find her in the ambulances or among the hundreds of people who had made their way out of the venue.

“I went around, searched, I asked everyone, I showed photographs. No one saw anything, no one could say anything,” he said.

As the death toll mounted, Pogadaev scoured hospitals in Moscow and the Moscow region.

His wife was not among the 154 reported injured nor on the list of 50 of the 133 fatalities who authorities had already identified, he said.

The Moscow region’s emergency situations ministry posted a video on Sunday showing equipment dismantling the damaged music venue to give rescuers access.

Putin called the attack “a bloody, barbaric terrorist act” and said Russian authorities captured the four suspects as they were trying to escape to Ukraine through a “window” prepared for them on the Ukrainian side of the border.

Russian media broadcast videos that apparently showed the detention and interrogation of the suspects, including one who told the cameras he was approached by an unidentified assistant to an Islamic preacher via a messaging app and paid to take part in the raid.

Kyiv strongly denied any involvement and the IS group’s Afghanistan affiliate claimed responsibility.

Putin did not mention IS in his speech and Kyiv accused him and other Russian politicians of falsely linking Ukraine to the assault to stoke fervour for Russia’s war in Ukraine, which recently entered its third year.

US intelligence officials said they had confirmed the IS affiliate’s claim.

The raid happened just days after Putin cemented his grip on the country for another six years in a vote that followed the harshest crackdown on dissent since the Soviet times.

– AAP