'Trolls' prompt UK debate over limits of internet free speech

If Twitter is the chirping chatterbox of the internet, trolls are its dark underground denizens.

The collision of the two is driving a debate in Britain about the scale of online hatred and the limits of internet free speech.

The furore erupted this week after several women went public about the sexually explicit and often luridly violent abuse they receive on Twitter from trolls - online bullies and provocateurs who send abusive messages.

Police are investigating a threat of rape and murder made to Labour Party lawmaker Stella Creasy. The graphically violent tweet was one of many Creasy received after supporting feminist campaigner Caroline Criado-Perez. Criado-Perez was sent a torrent of invective after she campaigned, successfully, for novelist Jane Austen to appear on a British banknote.

Two men have been arrested in connection with the Twitter threats, but have not been charged.

Police are also investigating threats against several female journalists after they received tweets warning them a bomb would detonate outside their homes at night.

Creasy and Criado-Perez are among a growing group who have decided to face down the abusers, re-tweeting their messages in an attempt to "name and shame" the offenders, and reporting the most threatening messages to police.