(Reuters) - Camouflage-clad gunmen opened fire at concert goers with automatic weapons on Friday, killing at least 60 people and injuring 145 more in an attack claimed by Islamic State militants.

In the deadliest attack in Russia since the 2004 Beslan school siege, gunmen sprayed civilians with bullets just before Soviet-era rock group "Picnic" was to perform to a full house at the 6,200-seat Crocus City Hall just west of Moscow.

Verified video showed people taking their seats in the hall, then rushing for the exits as repeated gunfire echoed above screams. Other videos showed men shooting at groups of people. Some victims lay motionless in pools of blood.

"Suddenly there were bangs behind us - shots. A burst of firing - I do not know what," one witness, who asked not to be identified by name, told Reuters.

"A stampede began. Everyone ran to the escalator," the witness said. "Everyone was screaming; everyone was running."

Russian investigators said the death toll was more than 60. Health officials said about 145 people were wounded, of which about 60 were in critical condition.

In the 2004 Beslan school siege, Islamist militants took more than 1,000 people, including hundreds of children, hostage.

Russian President Vladimir Putin was being updated by security chiefs about the situation, including from Alexander Bortnikov, the head of the Federal Security Service (FSB), the Kremlin said.

Russian investigators published pictures of a Kalashnikov automatic weapon, vests with multiple spare magazines and bags of spent bullet casings.