BEIJING — China’s once-a-decade leadership transition begins Thursday with all the pageantry, security, and behind-the-scenes political intrigue befitting the secretive ruling Communist Party’s most sensitive event.

By Wednesday, the usually crowded Tiananmen Square had been cleared, giving it an eerie, postapocalyptic feel. Activists had been chased out of the capital, and buildings across the city were draped in flags, flowers, and signs, all colored Communist red.