Chinese police have detained two people accused of inciting unrest in online postings, state media and a rights group said Wednesday in a sign of official concern over potential social instability.

Police in south China's Guangdong province detained a car owner for posting messages online trying to organise a bumper sticker campaign against rising vehicle licencing fees, the Global Times reported.

The owner, publicly identified only by an online name, hoped for 100 cars to parade around Yangjiang city to protest a proposed new 400 yuan ($65) annual vehicle licencing fee, the report said.

The driver printed bumper stickers depicting a clenched fist and reading: "To hell with the annual fee" and "Say no to corruption!", the state-owned paper said, and was detained by the authorities.

No protest took place and the licence fee has yet to be approved.

In the second case, police in Beijing took away Zhai Xiaobing on November 7 after he posted a joke about the collapse of the Great Hall of the People, where the Communist Party held a once-a-decade leadership transition this month, the Chinese Human Rights Defenders said.

Zhai wrote on Twitter that the collapse would happen in an upcoming film in the "Final Destination" horror franchise starting on November 8, the same day the party meeting started, the group said.

Twitter is blocked inside China and only web users who know how to circumvent the censors' "Great Firewall of China" can access it.

Zhai's disappearance has led several hundred people, including dissident artist Ai Weiwei, to sign an online petition for his release, the group said.

Both cases have raised concerns among lawyers, who say the detentions are not legal, while social media commentators have raised fears over curbs on civil liberties.

"The car owner shouldn't be detained as there was no parade or gathering," the Global Times quoted Xie Jiajun, a lawyer in Guangdong, as saying.

Police in Yangjiang refused to comment on the case when contacted by AFP. Beijing police were not immediately available for comment on Zhai's case.

The Chinese Human Rights Defenders said Zhai was being investigated for the crime of "spreading terrorist information".

The Nanfang Rural News identified the car owner as going by the name Zhu Fu 4321 and said the detention was for "inciting and planning illegal gatherings, protest marches and demonstrations and refusing to listen to warnings".

It added that Zhu Fu 4321's protest call came in late October, when police nationwide were on alert to quash all unrest ahead of the once-in-a-decade communist leadership handover.

The ruling party keeps a tight grip on dissent and China's vast security apparatus attempts to curb social unrest before it gathers momentum, while controlling reporting and online discussion of such events.

Academics estimate China saw 180,000 protests last year over a wide range of issues including corruption, government-backed land grabs, police brutality and unpaid wages.