Chinese dissident artist Ai Weiwei sent a defiant message to the authorities in Beijing on Tuesday in an opening address to the Stockholm Film Festival where he is on the jury -- but in spirit only.

The 56-year-old avant-garde artist, who is not allowed to leave China, is represented at the festival by an empty chair that he designed himself in an ironic reference to his inability to attend.

"I feel sorry I can't come. That's why I have designed and sent something symbolic," he said in a pre-recorded video message broadcast at a press conference.

"I hope it can give some kind of statement on the way authorities can limit freedom of speech, can limit basic human rights for artists to travel or participate in cultural activities -- very ruthlessly and with no explanation."

"I'm still living under a kind of soft detention. My passport is still in the authorities' hands."

His Ming Dynasty style chair -- shipped from Beijing for the 12-day event -- sat conspicuously between the other jury members in front of a cinema screen bearing the burly artist's bearded and somewhat haggard image.

The chair had one quirky addition: a curved bar preventing anyone from sitting on it.

Ai, known as much for his human rights activism as his art, was detained for 81 days in 2011 during a roundup of activists, and was accused of massive tax fraud.

He was one of many government critics who were jailed or detained or disappeared into police custody in early 2011, when calls for anti-government protests in China -- echoing those during the Arab Spring revolts -- rattled the authorities.

Film a powerful way to expose corruption

Since his release, he has been prevented from leaving the country.

In his video message Tuesday he said film was a powerful way to communicate and expose corruption.

"In the Internet age especially documentary films have made a great appearance in talking about issues, discussing truths, and also revealing secrets about those in power... especially when the power is not elected by the people."

He said that when he was interrogated by Chinese authorities and argued with them about freedom of speech and human rights, one of his interrogators said "you must watch too many Hollywood movies!"

"It sounds like a joke but maybe it's true since our history, our stories, our imaginations are made for films and for other people to watch and this is beautiful."

Ai makes wide use of social platforms like Twitter, where he has a huge following. Last year, he produced a version of the widely imitated South Korean hit Gangnam Style, pulling out a pair of handcuffs to symbolise the authorities' efforts to silence him.

The video was quickly censored.

Festival director Git Scheynius said Ai has the courage to stand up for "freedom" which is the theme of this year's festival in Stockholm.

"I think most people listening to him think 'wow, this is someone who is not afraid of anything' -- his courage is just wonderful," she told AFP.

She said however she was under no illusions that Chinese authorities would pay attention to the festival but hoped it provided food for thought.

"I hope they will listen to him and think that the (empty) chair says something to those people who want censorship in China," she said.

"At least somebody's courage can make them think. Nobody can imprison thinking."

But the authorities in China may be able to prevent Ai from carrying out his jury duties as it was still unclear Tuesday whether the festival could find a way of getting the films to him.

Not missing a chance to provoke, the artist signed off his video broadcast with "OK, I take off my clothes... be happy!" and stripped down to the waist.

The film festival runs through November 17.