People were seen screaming and taking cover in the first moments of the terrorist attack at Crocus City Hall on Friday.

Exclusive footage shows people hiding behind seats and trying to leave the concert venue building through stairways and fire exits amid the sound of gunfire.

Late on Friday, unidentified individuals reportedly dressed in military fatigues opened fire at Crocus City Hall, a 6,000-seat multipurpose concert hall venue where a Russian rock band was set to perform. A massive fire broke out in the building shortly after.

According to the Russian Investigative Committee, the attack left 133 people dead, with 152 injured, at time of publication. In turn, the Ministry of Health reported that as of Sunday, 110 were being treated in hospitals, and 32 had been released.

The Ministry of Emergency Situations said that rescuers were still clearing the rubble and the collapsed structures inside the building.

The Russian Investigative Committee opened a criminal case under Article 205 of the Criminal Code of the Russian Federation ('Terrorism').

Russian President Vladimir Putin declared a day of mourning and described it as a 'barbaric terrorist act' in a televised address on Saturday, adding that all four suspected perpetrators had been found and detained.

“They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them from the Ukrainian side to cross the state border,” Putin claimed.

Russia’s Federal Security Service (FSB) reported that the suspects had planned to cross the Russian-Ukrainian border and 'had relevant contacts on the Ukrainian side'.

The Russian Investigative Committee said that special services had detained four suspects in the Bryansk region, not far from the border with Ukraine.

On Friday, the Ukrainian Foreign Ministry said it 'categorically denies' all accusations of any involvement in the shooting.

The US and other Western countries attributed the attack to Afghan-based Islamic State affiliate 'ISIS-Khorasan' ('Isis-K'), citing an online channel they said was associated with the group.

The UN press service reported on Friday that Secretary-General Antonio Guterres condemned the attack on the concert hall 'in the strongest possible terms'.

“The Secretary-General expresses his deepest condolences to the families of the victims, the people and the government of the Russian Federation,” the statement added.

A number of foreign countries, including the United States, Belarus and Turkey, expressed condolences in connection with the shooting in the Moscow region.