President Xi Jinping extended condolences on Saturday to Russian President Vladimir Putin over the terrorist attack that caused heavy casualties on Friday near Moscow.

In his message, Xi said he was shocked to learn that the serious terrorist attack had occurred at a concert hall in Russia's Moscow Oblast.

Xi, on behalf of the Chinese government and the Chinese people, expressed his deep condolences for the victims, and extended his sincere sympathy to the injured and the bereaved families.

He emphasized that China opposes all forms of terrorism, strongly condemns terrorist attacks, and firmly supports the Russian government's efforts to maintain national security and stability.

The attack by gunmen on Friday evening at the Crocus City Hall northwest of Moscow claimed the lives of at least 137 people, according to Russian authorities.

Anastasia Rodionova, a survivor of the attack, told China Daily on Saturday that the assailants were "gunning down everyone methodically in silence" inside the venue on Friday night.

"We went down to some kind of ground floor, some dark room, and I saw only 'exit' word shining in the darkness, and we just did not know whether to run or not. Who is there in the dark? What is there in the dark?" she said after she laid flowers at the Crocus City Hall on Saturday morning. The woman was quickly surrounded by international media as she tried to describe what happened on the night of terror.

"China is deeply shocked by the heavy casualties inflicted by the severe terrorist attack," the Chinese Foreign Ministry said in an online statement, expressing deep condolences for the victims and sincere sympathy to the injured and bereaved.

"China opposes all forms of terrorism, strongly condemns terrorist attacks, and firmly supports Russia in safeguarding its national security and stability," the statement added.

According to Russian state media Tass, four suspects were detained in the Bryansk region and taken to Moscow and were in the custody of Russia's Investigative Committee.

One suspect said he was promised a reward of 500,000 roubles ($5,400), according to a video posted on RT Editor-in-Chief Margarita Simonyan's Telegram channel on Saturday.

The detainee added that half the money had been transferred to his card, while he had been promised he would receive the other half later. He lost the card while trying to escape from law enforcement officers.

The terrorist group ISIS claimed responsibility for the deadly incident.