A photograph taken in Moscow on March 23, 2024, shows the flag of Russia flying at half-mast on top of the Council of the Federation building, a after a gun attack on the Crocus City Hall in Krasnogorsk. (PHOTO / AFP)

MOSCOW - Russia lowered flags to half-mast on Sunday 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 133 people dead, including three children, and more than 150 were 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."

Putin said all four gunmen in the attack were detained, adding that all perpetrators, organizers and financiers of this crime will be punished

People laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men burst in on Friday just before Soviet-era rock group Picnic was to perform.

The men fired their automatic weapons in short bursts at terrified civilians who fell screaming in a hail off bullets.

It was the deadliest attack on Russian territory since the 2004 Beslan school siege, when Islamist militants took more than 1,000 people, including hundreds of children, hostage.

Long lines formed in Moscow on Saturday to donate blood.

In the southwestern city of Voronezh, people were laying flowers and lighting candles at a monument to children who died there in a World War Two bombing, in solidarity with those who died in the attack near Moscow.

"We, like the whole country, are with you," the governor of the Voronezh region, Alexander Gusev, said on the Telegram messaging app.

This photo taken in Moscow, Russia on March 23, 2024 shows screens displaying Russian President Vladimir Putin speaking in a televised address to the nation. (PHOTO /XINHUA)

Putin said all four gunmen in the attack were detained, adding that all perpetrators, organizers and financiers of this crime will be punished.

"Whoever they are, whoever guides them. I repeat, we will identify and punish everyone who stands behind the terrorists, who prepared this atrocity. This is a strike against Russia, against our people," Putin said.

The FSB said earlier that 11 people had been detained, including four who were directly involved in the terrorist attack, adding that the criminals intended to cross the Russian-Ukrainian border after the attack.

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Vyacheslav Volodin, chairman of the Russian State Duma, said those behind this horrific terrorist attack must be "eradicated."

Dmitry Medvedev, deputy chairman of the Russian Security Council, stressed that those involved must face retribution, vowing to find and eliminate the attackers.

Following the attack, security measures have been tightened across Russia. Security checks at Moscow's airports and train, subway and bus stations have also intensified. Moscow and other regions have canceled all sports, cultural and other large public events.

A woman mourns at a makeshift memorial in front of the Crocus City Hall, a day after a gun attack in Krasnogorsk, outside Moscow, on March 23, 2024. (PHOTO / AFP)

State Duma Deputy Chairman Pyotr Tolstoy suggested the attack should not be viewed separately from the special military operation and Russia's confrontation with the "collective West." "This terrible day will only force us to unite stronger to fight until victory," he added.

In response to the tragedy, spontaneous vigils were organized in several Russian cities. Electronic billboards in many cities displayed images of candles in mourning of those killed in the attack.

A Moscow resident named Alexey said he was deeply shocked upon learning about the incident last night and hopes those responsible will be swiftly apprehended.

"I feel very heartbroken and upset," said Moscow resident Nadezhda, who lives near the site of the incident. "It's a horrific deed to launch attacks on civilians."

United Nations Secretary-General Antonio Guterres strongly condemned the terrorist act, offering condolences to the victims' families, the Russian people and the government

Analysts note that this is one of the severe terrorist attacks Russia has faced in over a decade.

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Based on online photos and videos, political scientist Sergei Razgulyaev said that the attackers appeared professional, "like mercenaries."

Alexey Filatov, a veteran of Russian special forces unit Alpha Group, said that judging from eyewitness accounts and circulating videos, the attackers had long planned their assault, armed not just with firearms but also explosives.

Maria Zakharova, a spokeswoman for the Russian Foreign Ministry, called for international condemnation of the heinous crime.

The White House said on Friday night that there was no indication Ukraine or Ukrainians were involved in the attack and that it would continue to monitor the situation.

Cars drive past an advertising screen on the facade of a building displaying an image of a lit candle and the slogan "(We) Mourn 22.03.24" in Moscow on March 23, 2024, a day after a gun attack on the Crocus City Hall in Russia's Krasnogorsk. (PHOTO / AFP)

Zakharova said the US conclusions raised questions, urging the United States to share any reliable information with Russia.

Advisor to the head of the Ukrainian Presidential Office Mikhail Podolyak and the Ukrainian Foreign Ministry both rejected allegations of Ukraine's involvement in the attack.

United Nations Secretary-General Antonio Guterres and other world leaders strongly condemned the terrorist act and offered condolences to the victims' families, the Russian people and the government

United Nations Secretary-General Antonio Guterres and other world leaders strongly condemned the terrorist act and offered condolences to the victims' families, the Russian people and the government.

The United Nations Security Council said in a statement on Friday that it condemned "in the strongest terms the heinous and cowardly terrorist attack," underlining the need to "hold perpetrators, organizers, financiers and sponsors of these reprehensible acts of terrorism accountable and bring them to justice."

French President Emmanuel Macron strongly condemned the "terrorist attack," said the French presidential office on Friday, expressing solidarity with the victims, their families and all the Russian people.

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Italian Prime Minister Giorgia Meloni expressed in a statement on Friday the "firm and total condemnation of the Italian government for this heinous act of terrorism."

British Foreign Secretary David Cameron said on Saturday that Britain "condemns in the strongest terms" the deadly terrorist attack at the Crocus City Hall near Moscow.

"We offer our heartfelt condolences and express our deepest sympathy to the families of the many victims," he said in a statement on X, formerly Twitter.

In a message of condolences to Putin on Saturday, Mongolian President Ukhnaa Khurelsukh said his people "are deeply saddened to learn about the brutal terrorist attack that caused the loss of many innocent lives," expressing solidarity with the Russian people.

The US government had concerns about a potential terrorist attack in and around Moscow earlier in March, and the US State Department had issued a warning to Americans, said US National Security Council Coordinator for Strategic Communications John Kirby during an appearance on Fox News channel on Friday.

Smoke from fire rises above the burning Crocus City Hall concert venue following a shooting incident in the northwest of Moscow, Russia, on March 22, 2024. (PHOTO / XINHUA)

"We don't know right now the degree to which that warning and this attack are connected," he said, adding that "we have no indication, no sense at all, that the Ukrainians were involved in this in any way, shape or form."

Pakistan expressed its deepest sympathies with the families of the victims, the country's Foreign Ministry said in a late-night statement on Friday.

Narendra Modi, prime minister of India, called the tragedy a “heinous terrorist attack” and that India “stands in solidarity with the government and the people of the Russian Federation in this hour of grief”.

The Ministry of Foreign Affairs of Saudi Arabia slammed what it called “the armed terrorist attack”, stressing the “importance of fighting and combating all forms of extremism and terrorism”. 

The United Arab Emirates also condemned “these criminal acts” and its “permanent rejection of all forms of violence and terrorism aimed at undermining security and stability in contravention of international law”. A similar statement was issued by Qatar, reiterating its “firm position on rejecting violence and terrorism, regardless of motives and reasons”. 

Jordan Deputy Prime Minister and Foreign Minister Ayman Safadi on Saturday condemned the terrorist attack. King Abdullah has expressed condolences to Putin over the victims of the terrorist attack.

Malaysia’s Ministry of Foreign Affairs reaffirmed its stance in “rejecting terrorism and violent extremism in all forms and manifestations”, and “continues to underscore the urgent need for a concerted international effort to eliminate the scourge of terrorism in a comprehensive and effective manner”. 

The Philippines’ Department of Foreign Affairs also slammed the attacks and extended its sympathies to the bereaved families while Singapore’s Ministry of Foreign Affairs condemned the latest violence, saying that the attack “is a stark reminder of the threat that terrorism continues to pose to all countries” as it extended its condolences to the bereaved families and wishes for the injured a swift recovery.

With a report from Jan Yumul in Hong Kong