At least 133 people were killed, scores of others after several gunmen wearing combat attire opened fire at a concert hall in Russia's capital Moscow on Friday.
"Unknown people opened fire at the Crocus City Hall. The evacuation of people is ongoing," emergency services told TASS news agency. RIA Novosti agency reported wounded victims after "automatic gunfire," citing its journalist at the scene.
Russian news reports said that the assailants also used explosives, causing a massive blaze at the Crocus City Hall on the western edge of Moscow. Video posted on social media showed huge plumes of black smoke rising over the building.
The attack took place as crowds gathered for a performance by Picnic, a famous Russian rock band.
The musicians were not injured during the shooting, law enforcement agencies said.
Andrey Vorobyov, the governor of the Moscow region, announced establishing a task force to address the situation.
"A terrible tragedy occurred in the shopping center Crocus City today," Moscow Mayor Sergei Sobyanin said.
"I am sorry for the loved ones of the victims."
Sobyanin said all necessary assistance would be provided to those injured during the incident.
In one unverified video posted on social media, men with automatic weapons were shown firing repeatedly at screaming civilians, including women, who were cowering below what looked like an entrance sign to "Crocus City Hall."
Other video footage showed several people lying motionless in pools of blood outside the hall. Reuters was unable to immediately verify the footage.
The Investigative Committee said it detained 11 people, including four perpetrators, in the border region of Bryansk on their way to Ukraine.
Separately, the Federal Security Force (FSB) said it learned that a handler was waiting for the group on the Ukrainian side of the border to facilitate the passage and provide cover.
"Trying to escape, the terrorists were heading towards the Russian-Ukrainian border, planned to cross out, and had contacts on the Ukrainian side," it said in a statement.
The FSB said the group offered resistance, it had to shoot the tires to stop them, and now the group is being transported to Moscow for further investigation.
According to authorities, a main highway leading to Ukraine was closed, and other important roads were being thoroughly monitored during the detention operation.
The attack comes days after President Vladimir Putin cemented his grip on the country in a highly orchestrated electoral landslide.
Russian authorities said security was tightened at Moscow's airports and railway stations, while the Moscow mayor canceled all mass gatherings scheduled for the weekend.
The attack followed a statement issued earlier this month by the U.S. Embassy in Moscow that urged the Americans to avoid crowded places in the Russian capital in view of an imminent attack, a warning that was repeated by several other Western embassies.
Putin, who extended his grip on Russia for another six years in the March 15-17 presidential vote after a sweeping crackdown on dissent, earlier this week denounced the Western warnings as an attempt to intimidate Russians.
Russia was shaken by a series of deadly terror attacks in the early 2000s during the fighting with separatists in the Russian province of Chechnya.
In October 2002, Chechen militants took about 800 people hostage at a Moscow theater. Two days later, Russian special forces stormed the building and 129 hostages and 41 Chechen fighters died, most of them from the effects of narcotic gas Russian forces used to subdue the attackers.
In September 2004, about 30 Chechen militants seized a school in Beslan in southern Russia taking hundreds of hostages. The siege ended in a bloodbath two days later and more than 330 people, about half of them children, were killed.
