Chinese police have interrogated Communist Party veterans who publicly called for the ouster of the country's top security official ahead of a leadership change, two of the party members said Thursday.

The 16 retired officials from Yunnan province in southwest China called last week for the removal of Zhou Yongkang over his ties with ousted party leader Bo Xilai, in a rare open letter posted on the Internet.

Zhou, ranked ninth in the party hierarchy, is known as a hardliner and is said to have supported Bo, who was stripped of his party positions earlier this year in the nation's biggest political scandal in decades.

The open letter, posted on overseas websites, came ahead of a once-in-a-decade handover of power in China, where open dissent or organised criticism of top leaders is usually severely punished.

Since it was published, police in Yunnan province's Zhaotong city have called in lead authors Yu Yongqing and Zhao Zhengrong for questioning, according to two signatories of the letter who asked not to be named.

"The police told them that they shouldn't have put the letter on the Internet, they said that as old veteran Communist Party members they should know better," one of the signatories of the letter told AFP.

"The police were not menacing, they were very rational," he said, asking that his name not be used out of concerns for his safety.

Yu and Zhao, like the other signatories of the letter, are in their late 70s or early 80s.

As head of the party's Politics and Law Commission, Zhou is tasked with maintaining social stability and oversees a budget this year of $111.6 billion -- higher than China's declared military budget.

He is due to stand down at a party congress scheduled for later this year, but the letter called for his early removal to send a signal to the public on which direction the nation was heading.

It accused Zhou of overseeing a crackdown on crime and corruption in Chongqing, the city Bo oversaw before his downfall this year, that ignored the rule of law, involved the widespread use of torture and targeted Bo's political enemies.

Analysts say Bo's removal is indicative of a major split at the helm of the ruling party between hardliners advocating a return to latent Maoism and reformers who favour democratic reforms aimed at establishing the rule of law.

There is no indication that Zhou will be removed early. Last week he was named as a delegate to the upcoming party congress.