During Prime Minister's Questions today she criticised an online hate campaign that had been launched against the BBC's political editor Laura Kuenssberg .
Ms Allan called on Prime Minister David Cameron to help preserve people's right to free speech, safe from attack.
A petition by campaign group 38 Degrees had called for the BBC to sack Ms Kuenssberg over alleged biased reporting on Labour leader Jeremy Corbyn.
But the petition was taken down after it became a focal point for sexist and misogynist abuse.
Speaking in the House of Commons, Ms Allan said: "Respected journalist Laura Kuenssberg has been subjected to an online hate campaign that appears to be a sexist witchhunt to silence her.
"Increasingly this is a tool used against people in public life by those who take an opposing view.
"Will my Right Honourable Friend condemn this kind of harassment and will he work with media and social media platforms to preserve the right to speak freely without intimidation or hate?"
Mr Cameron agreed with Ms Allan's sentiments and condemned the anonymous abuse that takes place online.
He said: "We must be able to speak freely and we must have a robust and lively democracy.
"But some of the things people say on Twitter, knowing that they are in some way anonymous, are frankly appalling and people should be ashamed of some of the sexist bullying that often takes place."
The majority of those signing and supporting the petition for Ms Kuenssberg's resignation earlier this week expressed concerns about what they believed was biased reporting.
But many of the supporters on social media were seen to use abusive and sexist language calling for the BBC's first female political editor to go.
It was eventually taken down with the agreement of its original poster.
David Babbs, director of 38 degrees, said: "I am really concerned that a petition hosted on the 38 Degrees website has been hijacked, and used as a focal point for sexist and hateful abuse made towards Laura Kuenssberg on Twitter."