“We stand with you,” he said.
Australian Brenton Tarrant, 28, was charged on Saturday over the deadly attack, which also left 50 people injured.
Political leaders are grappling with the threat of far-right extremism in the wake of the killings.
Police Chief Commissioner Graham Ashton told media at the Preston mosque police were monitoring dozens of people with alt-right connections.
At the Australia Islamic Centre in Newport, Richmond resident Bronwyn Morkhan said it was more important now to show support for the Muslim community.
“We must all gain an understanding of one another,” she said. “We’ve been told we can come back at any time and I will do that.”
Police Inspector Peter O’Connor said he was humbled to see so many attend the open day, with hundreds, if not thousands, pouring into the prayer room.
“It’s great to see people of all types of backgrounds coming in and bringing their children,” he said.
“At the end of the day, it’s a church like any other, a place for people to gather and reflect.”
Mosque secretary Azzam Bararat said the Muslim community was in mourning after the attack in New Zealand.
But he said he felt humbled and surprised to see so many hundreds of people attending the open day.
“Honestly, we didn’t expect so many people,” he said. “It is very nice to have everyone coming together to show the spirit of the Australian community.
“We’re very upset; we are a peace-loving community.”
Westgate Migrant Resource Centre president Dimitrios Avgoulis, 90, said it was incredible to watch the seeds of equality that he’d help plant decades earlier grow.
“Every day you learn. I’m 90 years old and every day I learn more and that’s what today is all about,” he said. “This is the seed that we planted. It’s working.”
Williamstown Reverend Ron Browning said it was a proud day to see people of all faiths turn out to support the Muslim community.
“Solidarity is so important after what’s happened in New Zealand,” he said.
Former Victorian premier Steve Bracks has a longstanding history and friendship with those at the Newport mosque.
He said most people involved in the mosque hail from a place called Mish Mish, north of Beirut.
“I’ve visited the village and the people are so friendly,” he said.
“They’re great people and we want to show them support during this difficult time. The perpetrator of evil in New Zealand has caused us all to come together.”
Cards left at the mosque spoke of sadness over the attacks and solidarity with the Muslim community.
“To our Muslim friends and community. We want you to know we stand with you, in your grief.
We are so very sad that this unforgivable crime has happened to you,” one handwritten letter said.
“We hope that you know that there are many Australians who feel so deeply ashamed of these terror attacks on your culture, religion and community.
“We feel for you and offer our deepest condolences. There is no room for hate in this world.”
Visiting the Newport mosque, federal Opposition Leader Bill Shorten took the opportunity to attack hate speech.
“If you create a swamp of hate speech, then you cannot disown what crawls out of the swamp,” he said.
Mr Shorten spent more than an hour meeting Muslim leaders and Melbourne residents who turned up to show their support during the mosque open day.
He called the New Zealand attack an “unspeakable evil” and sympathised with Muslim families across the country now struggling to explain to their children that they are safe.
Millions of Australians were expressing solidarity ”after the dreadful, dreadful murderous killing of innocent worshipers … I never thought I’d utter the words Australian terrorist,” Mr Shorten said. “But it takes a terrible event like we’ve seen in Christchurch to stop and think about who we are as a people.”
Preston Mosque regular Siham Merhi said arriving at her usual place of prayer took on special significance after Friday’s attack.
“We’ve been devastated for the past couple of days just seeing what’s happened to our fellow human beings,” she said. “They were at their most vulnerable.”
Ms Merhi said visitor numbers at her mosque’s open day were much bigger than in previous years.
She said people had offered apologies and their condolences after the attack. Those gestures had added to her sense of community.
“It’s going to unite us more than ever. He didn’t achieve anything by doing this at all.”
Carolyn Webb is a reporter for The Age.