Born when a Qing dynasty emperor was on the throne, the man who helped invent the Pinyin system used for writing Chinese worldwide turned 109 on Tuesday. But Zhou Youguang's outspoken support for democracy means his works are still censored by the ruling Communist party.

"After 30 years of economic reform, China still needs to take the path of democracy," Zhou told AFP in an interview, his wrinkled face topped with a patch of white hair. "It's the only path. I have always believed that."

Zhou is commonly known as the "father of Pinyin", a system for transliterating Chinese characters into the Roman alphabet introduced in the 1950s and now used by hundreds of millions of language learners in China, as well as abroad.

But in his cramped third-floor apartment in Beijing, where dog-eared books -- including dozens by Zhou himself -- line the walls, the writer was modest about his achievements.

"I don't have any feeling of pride. I don't think I've achieved very much," he said, speaking lucidly but slowly and with obvious effort. "My birthday is of no importance at all."

Born to an aristocratic family in 1906, Zhou experienced the last years of the Qing dynasty and its revolutionary overthrow, before studying at elite universities in Shanghai and Japan.

When Japan launched a full-scale invasion of China in 1937, Zhou moved with his wife and two children to the central city of Chongqing, where he endured constant air raids but made contacts with leaders in the then comparatively weak Communist party.

After Japan's defeat he avoided China's civil war between the Communists and Nationalists by going to work for a Chinese bank on Wall Street, twice meeting Albert Einstein while visiting friends at Princeton.

But following the Communist victory in 1949, Zhou returned home to teach economics and became a close associate of the party's number two, Zhou Enlai.

"I came back for two reasons: because I thought the country had been liberated, and had a new hope. Also, because my mother was in China," he wrote in a 2012 autobiography.

He was attracted to Mao Zedong's Communists because "at that time they promoted themselves as democrats", he wrote.

- 'Pessimists tend to die' -

An amateur linguist who had taught himself some Esperanto, Zhou was assigned in 1955 to co-chair a committee tasked with increasing literacy by reforming the Chinese language.

He eventually backed a system based on one developed in the Soviet Union, using Roman letters to represent pronunciation alongside marks to indicate tone.

The proposal, named Pinyin -- "putting together sounds" -- is used in schools across China and has been instrumental in boosting the country's literacy rate from around 20 percent in the 1950s to more than 90 percent today.

Though systems for transcribing Mandarin into the Roman alphabet already existed -- including Wade-Giles, produced by two British diplomats in the 19th century -- Pinyin is regarded as simpler.

"With Chinese characters, you can't tell the pronunciation just by looking. So Pinyin was useful in teaching," said Luo Weidong, a professor at Beijing Language and Culture University. "Pinyin made a big contribution to the literacy movement in China."

In recent decades, Pinyin has become key to the easy creation of Chinese characters on computers.

But Zhou's contributions did not save him from the chaos of Mao's decade-long Cultural Revolution from 1966, during which intellectuals were persecuted.

Zhou, then in his 60s, was sent to work at a labour camp in faraway Ningxia for more than two years, separated from his wife and son.

"I had never slept on an earth bed before," he wrote of the experience, adding: "When you encounter difficulties, you need to be optimistic. The pessimists tend to die."

He has described the two decades from 1960 to 1980 as "wasted", adding: "In all honesty I haven't got anything good to say about Mao Zedong."

He has a higher opinion of Mao's successor Deng Xiaoping, who launched market-style reforms which helped transform China into the world's second-largest economy.

But since retiring aged 85, Zhou has written dozens of books arguing that Deng's reforms are insufficient without political change.

"Chinese people becoming rich isn't important," he said. "Human progress is ultimately progress towards democracy."

- 'Problem with the system' -

Zhou, probably China's oldest dissenter, marked his birthday with friends and family, enjoying dishes including braised sea cucumber and a date and mushroom soup, his editor Ye Fang said.

Sleeping takes up an increasing proportion of his time as his health flags, but he is still a voracious reader. Confucius and Socrates remain his favourite thinkers.

Age appears to have been no barrier to a harsh crackdown against critics of the party overseen by China's current President Xi Jinping.

Scores of journalists, lawyers and academics have been arrested and dozens jailed, among them 71-year-old journalist Gao Yu, tried last year for leaking state secrets, and writer Tie Liu, 81, detained since September.

Zhou's books have also come under more intense scrutiny, with topics which could be tackled just a few years ago now taboo.

Censors demanded that his latest book, due out next month, be purged of some references to anti-intellectual movements, as well as a 1950s famine which killed tens of millions as a result of Mao's "Great Leap Forward".

"The restrictions on publishing have got tighter. No one knows if it's a short-term thing, or a long-term change," said Ye.

Sitting beneath peeling paint in his flat, Zhou said the leader was not the issue.

"I don't think it's a problem of individuals," he said. "It's a problem with the system. We don't have freedom of speech in China."