By Farah Master

LONDON (Reuters Life!) - A twitter opera about nuns, demons and birds will be performed in London this weekend after Britain's Royal Opera House (ROH) composed a libretto from the popular microblogging site.

As part of an attempt to appeal to a wider audience, the ROH asked the public three weeks ago to "tweet" their contributions help create a story.

So far, around 900 people have used the twitter website to write 140-character tweets, and the volume of tweets has generated enough content for a seven-act opera, said ROH spokeswoman Sara Parsons.

"When pigs do fly, these birds will cry, and forever more leave torture to the devil. Bring me a pig!" was one of the contributions sent online.

Extracts of the twitter opera, lasting 20 minutes each, will be performed on Saturday and Sunday at the ROH in London, with the twitter contribution forum still open for public suggestions at www.twitter.com/youropera

"It's about getting people to contribute to their own original story," said Parsons.

The twitter opera, performed by two singers, will use familiar opera tunes as well as original music by composer Helen Porter.

(Editing by Steve Addison)