Hundreds of Chinese have flooded US President Barack Obama's Google+ page, apparently taking advantage of a glitch in China's censorship system to post about human rights and green cards.

Google+ -- the US Internet giant's social networking site -- has been unavailable in China since it was launched last year, apparently blocked by the nation's strict censorship system, widely dubbed "the Great Firewall of China."

But over the past few days, Chinese netizens say they have unexpectedly managed to get onto the site, and have decided to concentrate their presence on Obama's page.

"Many people don't understand the meaning why all Chinese are coming here. We envy American people their democracy and freedom!" one netizen said in English under Obama's latest posting about his campaign's bumper stickers.

The comments centre around freedom of expression and human rights, as well as more mundane issues such as how to get US green cards.

"We are not barbaric people, we are just suppressed," one netizen wrote, while others called on the US president to "come and liberate China".

It is hard to prove if all those commenting are from China, but most write in simplified Mandarin characters -- used in the mainland as opposed to Hong Kong -- and expressions are similar to those found on the nation's microblogs.

It is also difficult to determine what glitch in the Firewall they are taking advantage of.

On Sunday, Google+ appeared to be available in China on some people's mobile devices but not on fixed computers -- a fact confirmed by some netizens on Obama's page.

Others in China, however, were still unable to access the site, which remained blocked along with Facebook, Twitter and YouTube on their mobiles.

Some netizens urged Obama to help free activists such as blind lawyer Chen Guangcheng, who is currently under house arrest, or Liu Xiaobo, the jailed Nobel Peace Prize winner.

Many of those posting comments also enquired about green cards -- US permanent residency certificates -- asking Obama to send them one along with his campaign stickers.

One called on the US president to join China's ruling Communist Party.

"We will give you a big red flower, which you can wear on your chest, and honourably give you a party certificate (completely free membership for the first year)," said the online user, named Duke Dai.