Russia has lowered flags to half-mast for a day of mourning after scores of people were gunned down with automatic weapons at a rock concert outside Moscow in the deadliest attack inside Russia for two decades.
President Vladimir Putin declared a national day of mourning after pledging to track down and punish all those behind the attack, which left at least 133 people dead, including three children, and more than 150 injured.
"I express my deep, sincere condolences to all those who lost their loved ones," Putin said in an address to the nation on Saturday, his first public comments on the attack.
"The whole country and our entire people are grieving with you."
Islamic State claimed responsibility for Friday's attack, but Putin has not publicly mentioned the militant group in connection with the attackers, who he said had been trying to escape to Ukraine.
He asserted that some on "the Ukrainian side" had prepared to spirit them across the border.
Ukraine has repeatedly denied any role in the attack, which Putin also blamed on "international terrorism".
People laid flowers at Crocus City Hall, the 6200-seat concert hall outside Moscow where four armed men burst in on Friday just before Soviet-era rock group Picnic was to perform its hit Afraid of Nothing.
The men fired their automatic weapons in short bursts at terrified civilians who fell screaming in a hail of bullets.
It was the deadliest attack on Russian territory since the 2004 Beslan school siege, when Islamist militants took more than 1000 people hostage, including hundreds of children.
Long lines formed in Moscow to donate blood.
Blood banks said on Sunday they now had enough blood supplies for four to six months.
Across Moscow, billboards carried a picture of a single candle, the date of the attack and the words "we mourn".
Countries around the world have expressed horror at the attack and sent their condolences to the Russian people
Putin said 11 people had been detained, including the four gunmen who fled the concert hall and made their way to the Bryansk region, about 340km southwest of Moscow.
"They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them on the Ukrainian side to cross the state border," Putin said.
Russia's Federal Security Service (FSB) said the gunmen had contacts in Ukraine and were captured near the border.
The suspects have been brought to Moscow and could appear in court later in the day, according to local news agencies.
Putin ordered a full-scale invasion of Ukraine in February 2022, triggering a major European war after eight years of conflict in eastern Ukraine between Ukrainian forces on one side and pro-Russian Ukrainians and Russian proxies on the other.
Ukrainian President Volodymyr Zelenskiy said it was typical of Putin and "other thugs" to seek to divert blame.
Islamic State, the Islamist group that once sought control over swathes of Iraq and Syria, claimed responsibility for the attack, the group's Amaq agency said on Telegram.
On Saturday night, Islamic State released on its Telegram channels what it said was footage of the attack.
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 said in poor and highly accented Russian.
When asked why, he said "for money".
The man said he had been promised half a million roubles ($A7700).
One was shown answering questions through a Tajik translator.
Tajik President Emomali Rakhmon spoke to Putin about the attack
The White House said the US government shared information with Russia early this month about a planned attack in Moscow and issued a public advisory to Americans in Russia on March 7.
It said Islamic State bore sole responsibility for the attack.
"There was no Ukrainian involvement whatsoever," US National Security Council spokeswoman Adrienne Watson said.
