When the judge ordered a recess, Kasparov confronted prosecutor Gyulchekhra Ibragimova as she walked past him on her way out.
During the brief and tense exchange, Ibragimova told Kasparov she respected him but added he should have been playing chess rather than wasting his time in court.
"You are an amateur" in the courtroom, she told him.
A smiling Kasparov accused the prosecution of seeking "to replace the force of law with the law of force," and suggested she and other prosecutors were "selling the honor of your profession" by pursuing the case against Khodorkovsky.
A half-dozen guards, some with automatic rifles and pistols, stood by watching impassively.
Khodorkovsky and his co-defendant, former Yukos executive Platon Lebedev, witnessed the confrontation at close range from inside the glassed-in defendant's booth. Both grinned. Lebedev winked at one supporter.
Many of the roughly 30 spectators in the courtroom were Khodorkovsky supporters, and during the recess several shouted at prosecutors as they hurried down the stairs.
Kasparov, one of the most famous of the Russian opposition leaders, said before the trial began it was his "civic duty" to demonstrate support for Khodorkovsky.
