(Reuters) -Russia's Federal Security Service (FSB) said on Tuesday it had opened a criminal case against exiled Kremlin critic Mikhail Khodorkovsky, accusing him of creating a "terrorist organisation" and of plotting to violently seize power. The FSB said it was investigating more than 20 other people as part of the same case, including prominent dissident Vladimir Kara-Murza, former world chess champion Garry Kasparov, ex-prime minister Mikhail Kasyanov and leading economists Sergei Aleksashenko and Sergei Guriev.