"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," one Twitter user posted.
Australian Muslim leaders released a statement condemning the event, calling it a "despicable act."
Quilliam Foundation, a counter-extremism think tank, has been warned of increased jihadist attacks for for the past few years.
"[I]t's just the beginning for Australia in what could well be a number of attacks in the region," said Hara Rafiq, head of outreach at Quilliam Foundation. "This is something indicative to what we have coming."
The siege should serve as a warning to to the rest of the world," Rafiq told CNBC in a phone interview Monday. "Australia, in particular, has the worst problem of Jihadi foreign fighters in the U.K., per capita."
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The alleged gunman, Shiekh Haron, had not been linked have any direct links to ISIL, but he's linked to "their ideology that allows him to kill other people, be their Muslims or non-Muslims, to affect political change," Rafiq noted.