Russia lowered flags to half-mast on Sunday for a day of mourning and swiftly charged two suspects after gunmen killed scores of people at a concert outside Moscow in the deadliest attack inside Russia for two decades.

President Vladimir Putin declared a national day of mourning after pledging to punish all those behind the attack on Friday evening, in which 137 people were killed, including three children, and 182 were injured.

Over 100 people remained in hospital, some of them in a serious condition. Video footage showed a sombre-looking Putin lighting a candle at a church at his residence outside Moscow on Sunday evening to honour those who died.

Earlier on Sunday, people laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men burst in just before Soviet-era rock group Picnic was to perform. The men fired their automatic weapons in short bursts at civilians who fell screaming.

Islamic State has claimed responsibility for the attack, but Putin, who has yet to name those he thinks are responsible, has not publicly mentioned the Islamist militant group in connection with the attackers, who he said had been trying to escape to Ukraine. He said that some on "the Ukrainian side" had been prepared to spirit the gunmen across the border.

Ukraine has denied any role in the attack.

Moscow's Basmanny district court on Sunday charged two suspects with acts of terrorism in connection with the attack, naming them as Dalerdzhon Barotovich Mirzoyev and Saidakrami Murodali Rachabalizoda, according to Moscow courts' official Telegram channel.

It said Mirzoyev, a Tajik national, pleaded guilty to all charges and the court remanded him in pre-trial custody until May 22.

In video footage published by Russian media and Telegram channels with close ties to the Kremlin, one of the suspects said he was offered money to carry out the attack.

"I shot people," the suspect, his hands tied and his hair held by an interrogator, a black boot beneath his chin, said in poor and heavily accented Russian.

When asked why, he said: "For money." The man said he had been promised half a million roubles (a little over US$5,000). One was shown answering questions through a Tajik translator. (Reuters)