Many of the demonstrators were car industry workers, voicing their opposition to proposed Canadian workplace laws.
An organiser of the rally says they are afraid Canada's new conservative government will use new Australian workplace laws as a model for Canada.
Meanwhile, Mr Howard has addressed a joint sitting of the Canadian parliament, pointing out that Canada and Australia have a long history of being allies at war.
“And now together in response to the new and dangerous threat of terrorism in Afghanistan. And I pay tribute to the enormous contribution of the Canadian nation to the effort in Afghanistan. And I mourn the loss and the sadness of Canadian families in recent days.”
Officials say Mr Howard and Canada's Prime Minister Stephen Harper will hold talks on security and trade.