The ongoing hostage situation in Sydney has sparked an outpouring of solidarity on social media with the hashtag #illridewithyou trending worldwide.

Two Arabic flags, similar to those used by Islamic State, have been seen inside the Lindt cafe.

The Australian government and Muslim leaders have condemned the attack and called for unity.

Amid uncertainty about the hostage-taker's motives and fears of reprisals an Australian woman reportedly started the #illridewithyou hashtag to reassure Muslims who might feel threatened on public transport in the coming days.

@SirTessa wrote: "If you reg take the #373 bus b/w Coogee/MartinPl, wear religious attire, & don’t feel safe alone: I’ll ride with you. @ me for schedule."

Twitter users have responded to this in their thousands with many offering to accompany people in Sydney and other Australian cities.

@Eddieperfect wrote: "My family and I are heading in and out of the city by tram in melbs tomorrow. We'll stand by anybody #illridewithyou."

@karennaree tweeted: "#illridewithyou anytime, anyplace, anywhere. We won't be beaten by evil. Praying the hostages will be safe."

The hashtag has been tweeted over 90,000 times.

Elsewhere on social media a number of people have been criticised for tweeting selfies standing by the cordon around the Lindt cafe.

Passers-by and shoppers uploaded the images of themselves to Twitter with hashtags including #hostagesituationselfie.

Their actions have been condemned on the micro-blogging site.

@stillgray wrote: "People taking selfies at the area around the Sydney hostage situation. We have reached peak selfie and it is terrible."