Russia's Investigative Committee said the incident happened when five handcuffed defendants were escorted by two guards in a lift.
One of the defendants tried to strangle one of the guards, and the defendants managed to escape and seized the guards' weapons, the investigative body said in a statement.
Three law enforcement officials were wounded, said Petrova and Yevgeny Kubyshkin, a spokesman for the Russian national guard, which had an officer in the court.
The members of the GTA Gang, which took its name after a computer game, are charged with murders of vehicle drivers in Moscow, the Moscow and neighboring Kaluga regions in 2012-2014.
Three members of a gang accused of highway killings in the Russian capital were killed in a Moscow court after a shoot-out. She said that she saw a wounded female guard who's "face was all bloody".
Russian police officers and a man stand outside the Moscow Regional Court building outside Moscow after a shooting on August 1, 2017.
At least two other people, including a man, 45, and a woman, 40, within the courtroom were injured and hospitalized, the Moscow region health ministry said.
"Two are members of the so-called GTA Gang".
According to the court's spokeswoman Natalia Osipova, by now, the court has resumed its work.
A total of nine defendants, all from Central Asia, are on trial.