The Twitter hashtag (Hash) IllRideWithYou had been used more than 90,000 times by early Tuesday, as tweeters tried to allay fears of anti-Islamic attacks on Australia's streets.
Hostages were held for more than 16 hours inside Sydney's Lindt Chocolat Cafe by a gunman who had a flag bearing an Islamic declaration of faith that has been used in jihadi imagery. That prompted speculation that the siege could ignite retaliatory violence against Muslims.
Sydney resident Rachael Jacobs wrote on Facebook that she had seen a woman on the train remove her headscarf and offered to walk with her.
That spurred a Twitter campaign in which users offered to travel on public transit with those in Islamic dress who felt insecure. Users were encouraged to supply details of their travel routes to ensure their online gestures were practical.
Kristen Boschma, a social media manager in Melbourne, printed out a sticker with the hashtag and stuck it on her bag. Her photo of the sticker was retweeted hundreds of times.
She said she wanted to send a message of support "not just for the Muslim community, but for anyone who feels a bit scared or insecure about taking transport or being out and about."
Boschma said she hoped the siege would prove "galvanizing rather than polarizing" for Australia.
"We very much believe in looking out for our mates," she said. "And I think this situation has widened the definition of what is a mate."