Australia's national leaders have visited the Bourke Street Mall to honour victims killed in Friday's rampage, with Prime Minister Malcolm Turnbull saying all Australians stand in solidarity with mourners.
A shrine to the victims of the tragedy, which has claimed five lives including that of a three-month-old boy, has been set up on the steps of the former General Post Office in the mall.
Mourners have been leaving flowers and tributes at a site normally filled with shoppers.
Mr Turnbull joined Victorian Premier Daniel Andrews and senior police to lay flowers at the site and speak with emergency service workers and counsellors.
"All Australians are with you, the people of Melbourne, the people of Victoria in your grief. We are with you in solidarity, we are grieving with you, we are mourning those whose lives have been lost," Mr Turnbull said.
"We admire and thank the heroism of the first responders ... the bystanders who quickly moved to help those that had been injured."
The Prime Minister paid tribute to the "selflessness and solidarity" of all those who came to help, saying it showed the best of the Australian spirit.
"This is a sad day. It is so sad. The loss of our fellow Australians in such a shocking, wanton criminal attack is a tragedy," Mr Turnbull said.
Earlier, Opposition Leader Bill Shorten laid flowers at the scene, spoke to mourners and condemned the act.
"There is no satisfactory explanation, no set of circumstances which can condone this act of evil," Mr Shorten said.
"What is the world coming to?"
The federal Labor leader said he was shocked by the violation of a Melbourne icon and that there were questions to be asked.
Mr Andrews thanked Mr Turnbull for attending the Bourke Street memorial.
"Our hearts have been broken by this," the Premier said.
The federal government will make a contribution to the newly opened Bourke Street Fund for victims' families, Mr Turnbull said, and he and his wife, Lucy, would make a personal contribution.
Donations for the immediate families of those killed can be made via the website www.vic.gov.au/bourkestreet.html
For urgent support, call Lifeline on 13 11 14 for confidential 24/7 counselling and referrals.
Victoria Police is urging witnesses and victims requiring support to call the Victim's Support Helpline on 1800 819 817.
A vigil is being planned for Monday night at Federation Square.
