The world’s best-known advocate of freedom of the media took its message to the heart of Beijing this morning, making a pirate broadcast on Chinese radio exactly 12 hours to the minute before the start of the Olympics opening ceremony.

Paris-based Reporters Without Borders began broadcasting on local FM radio to several districts of Beijing at 8.08 a.m local time (0000 GMT), denouncing China’s grip on media and expression. The broadcast, in both English and Mandarin Chinese, while often indistinct, lasted for 20 minutes.

In the latest embarrassing breach of China’s massive security operation, the group used the FM 104.4 frequency to demand the release of political prisoners and the lifting of censorship. A voice at the start of the broadcast said: “China is the country of censorship, and this programme is our way of making fun of the Chinese authorities who still keep hundreds of journalists and Internet users in prison."

Vincent Brossel, a Paris-based spokesman for the group, said they timed the audacious on-air challenge to coincide with the final build-up to the Beijing Games opening on Friday evening. "It's 12 hours before the opening of the Olympics - a time when we want these voices heard." Members of RSF disrupted the Olympic torch lighting ceremony in Greece earlier this year, setting the stage for demonstrations throughout the relay.

The broadcast said: “It's our way of saying to them: Despite everything you do, here are the voices of people you want to silence and they are speaking in the heart of Beijing on the very first day of the Olympics.

"It's our way of saying: Whatever measures you take you will never be able to abolish the right to free speech."

The press freedom organisation said it was the first time since the Communist Party came to power in 1949 that a non-state radio station had broadcast in China. International Chinese-language radio station broadcasting on short wave could be heard in China but are jammed by the authorities.

Robert Menard, secretary general of the group, said the broadcast was a gesture of defiance to the Chinese authorities who hare keeping dozens of journalists and Internet users in prison. He said in the broadcast: “Despite everything, there are people who are going to be able to speak out about things you don't want the public to hear, in the very heart of Beijing. Regardless of the measures you take, you will not get rid of free speech."

He called for the release of prisoners of conscience and told China that its censorship would not work.

The Broadcast included interviews with Chinese human rights activists now living overseas. Yang Jianli, a former political prisoner, described prison conditions. “External pressure is essential to improve the situation of political prisoners.”

The numbers of those in jail for defying China’s censors is difficult to know, but Reporters Without Borders has estimated that roughly 100 journalists, Internet users and “cyberdissidents” are serving jail sentences for expressing their views.

Media openness has stirred controversy even before the start of the Games when it emerged that China’s cyber police had no intention of providing journalists with the free and unfettered access to the Internet that had been promised for the duration of the Olympics.

Some sites have been opened, notably that of Reporters Without Borders and Amnesty International as well as some Chinese-language newspapers based in Taiwan and Hong Kong whose stance irks the Chinese leadership. However, many more remain inaccessible behind the Great Firewall of China. Most sites that mention Tibet and the Falun Gong religious sect remain blocked – as do many blog hosting sites such as Typepad, which is used by The Times bloggers.

China introduced new rules from January 1 last year allowing foreign journalists to report more freely across most of China in the run-up to the Olympics, but these will expire when the games end.

Chinese journalists are careful to censor what they write to avoid angering the authorities. The outspoken Southern Weekend newspaper ran a long article on the issue of the suspected shoddy construction of the many schools that crumbled in the devastating earthquake in Sichuan in May. Beijing censors were furious and have penalized the newspaper, reducing by four on occasion the number of pages it is allowed.