BEIJING — In a rare sign of open opposition against two of China’s most powerful leaders, a group of retired Communist Party members have called for the resignation of the country’s security boss, Zhou Yongkang, and a top propaganda official, Liu Yunshan.

In a petition that began to circulate Wednesday on social media and overseas Web sites, the 16 signers — all of them retired midlevel officials in the southern province of Yunnan — accused Mr. Zhou of supporting the fallen party leader Bo Xilai, who was suspended from his posts amid reports that he helped cover up a murder and abused power.

Mr. Bo, who was the head of the city-state of Chongqing in southwestern China, advocated a return to old-style Communist propaganda and carried out a heavy-handed crackdown on organized crime. Although he was popular with many people for bolstering the economy, civil rights groups and lawyers said he ran a virtual police state.

“We still care about the future of the country,” said one of the signers, Wu Zhibo, 84, a former vice head of a vocational school in the city of Zhaotong.