Complex sentence: After the beginning of Gorbachev's perestroika he was released in 1987 under an amnesty and immediately resumed its human rights activities: proceeded to publish "Glasnost" magazine, criticizing the communist system.
Split into simpler sentences: After the beginning of Gorbachev's perestroika he was released in 1987 under an amnesty. He immediately resumed his human rights activities and proceeded to publish "Glasnost" magazine, criticizing the communist system.