Twitter's UK boss has personally apologised to women who have been attacked by "trolls" on the social networking site, vowing to do more to protect people from abuse.
Tony Wang, general manager of Twitter UK, posted a series of tweets today saying abuse was "simply not acceptable".
He wrote: "I personally apologise to the women who have experienced abuse on Twitter and for what they have gone through.
"The abuse they've received is simply not acceptable. It's not acceptable in the real world, and it's not acceptable on Twitter.
"There is more we can and will be doing to protect our users against abuse. That is our commitment."
His messages come as Twitter clarified its rules on abusive behaviour amid a growing backlash over a series of attacks.
The company has updated its rules to make it clear that abuse will not be tolerated and has put extra staff in place to handle reports of abuse, it said.
The move comes as Scotland Yard said its e-crime unit is investigating allegations by eight people of abuse on the microblogging site.
There has been growing concern over abuse on the site after three female journalists said they had been the subject of bomb threats, while two received threats of rape.
An online petition calling for Twitter to add a "report abuse" button to tweets has already attracted more than 124,000 signatures.