Catalans are asking Spain's Guardia Civil to apologise for a "joke" tweet, threatening combat vehicles on the streets if they vote for independence.

Nerves are high in Catalonia right now. The region in north east Spain, which has its own language, has plans for a referendum on independence, which its President Artur Mas says will be held in November next year. The central government in Madrid is having none of it - calling the vote unconstitutional, and insisting it will never happen. Social media in Catalonia is alive with debate on the subject, with the hashtag #sisi - calling for people to vote "yes" - trending. Opinion polls suggest a vote - if it's ever held - would be close.

Something of a social media hand grenade was first thrown into the mix last Friday, when a tweet went out from an account which seems to be run by members of Spain's Guardia Civil police force. It implied that they would be sent in if Catalonia voted for independence, and asked Catalans if they would prefer to have armed men marching down the main street in Barcelona, or armoured combat vehicles instead.

Image caption A Catalan independence rally in Barcelona in September this year

It was prefaced #humor, to indicate it was a joke, but for some, it was anything but. Catalans have a history of enmity towards the Guardia Civil because of their role in implementing Franco's crackdown on their regional identity.

"It is not funny at all. Praising military takeovers? Is this #humor?" tweeted Cristina Juesas Escudero, who teaches communication at University of the Basque Country, but is from Madrid herself. She was so outraged she set up an online petition calling on the Guardia Civil to apologise. The force say they have nothing to apologise for as the tweet was not from an official account - and they are unable to establish whether those tweeting from it are Guardia Civil or not. Since setting up the petition, Juesas Escudero says she has been harassed on Twitter, Facebook and online forums.

When it comes to the question of Catalan independence, there are often fierce and unpleasant exchanges on social media, says Martí Crespo, a journalist with the online Catalan news site VilaWeb. There is even a Twitter account - with 18,000 followers - which highlights examples of "anti-Catalan" views on social media.

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Reporting by Cordelia Hebblethwaite