Twitter has apologised to women attacked by "trolls" on the micro blogging website, as it updated its rules on abusive behaviour.
The social website's UK general manager Tony Wang on Saturday posted a series of tweets saying abuse was "simply not acceptable" after Britain's police said they were investigating allegations made by eight people.
It comes after three female journalists said they had been the subject of bomb threats, while two received threats of rape.
"I personally apologise to the women who have experienced abuse on Twitter and for what they have gone through," Wang wrote.
"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."
An online petition calling for Twitter to add a "report abuse" button to tweets has attracted more than 120,000 signatures.
In a message, Wang and Del Harvey, Twitter's senior director for trust and safety, said: "It comes down to this: people deserve to feel safe on Twitter."
They said the clarified rules make clear that Twitter will not tolerate abusive behaviour, and an "in-tweet" report button has been added so people can report abusive behaviour directly from a tweet.
"We want people to feel safe on Twitter, and we want the Twitter rules to send a clear message to anyone who thought that such behaviour was, or could ever be, acceptable," they wrote.
Al Jazeera's Phil Lavelle, reporting from London, said: "Part of that will be hiring more staff to sift through abuse - Twitter's vowing to do that too."
"Tackling the trolls may be long, time consuming process," he said.
Britain's Scotland Yard police headquarters said an investigation had been launched into eight allegations of "malicious communication" made on the site.
High-profile women in Britain have long complained of online harassment, but the issue reached front pages last week when feminist campaigner Caroline Criado-Perez met with a barrage of abuse after successfully lobbying for novelist Jane Austen to appear on a banknote.
The 29-year-old said on Saturday: "The current process is lengthy, complicated and impossible to use if you're under sustained attack like I have been.
"Right now, all the emphasis is on the victim, often under intense pressure, to report rather than for Twitter to track down the perpetrator and stop them.
"This will take time, investment and properly trained and paid staff - but it's crucial they get this right."
Two British men were arrested after Criado-Perez and two female lawmakers reported menacing tweets to police.