A Twitter campaign against Islamophobia in the wake of the hostage crisis in Sydney has gone viral, with thousands of netizens expressing their solidarity with Australian Muslims.

A woman started the hashtag #IllRideWithYou pledging to accompany Muslims wearing religious attire on public transportation to defuse potential episodes of intolerance.

The hashtag was taken up by thousands of netizens and was mentioned more than 40,000 times in a few hours, trending globally in the morning.

Meanwhile, a number of Australian Muslim organisations have expressed their "shock and horror" at the unfolding hostage situation in Sydney's Martin Place.

A gunman forced hostages inside a Lindt cafe to hold up a black Islamic flag in what is a suspected attack by a person "claiming political motivation", according to Australian Prime Minister Tony Abbott.

A statement from 50 Australian Muslim organisations said that the community's "thoughts go to the hostages and their loved ones" and that they "pray for their safety".

"The Australian Muslim community shares with fellow Sydneysiders their utter shock and horror at the unprecedented scenes emerging from the Lindt cafe in Martin Place this morning," the statement reads.

"We reject any attempt to take the innocent life of any human being, or to instil fear and terror into their hearts."