Organizers of the World Social Forum 2016 in Montreal say many invitees have had their visitor visa application denied by the government.
They say this could tarnish Canada's reputation as an open society.
Twenty-four hours before the start of the event, co-ordinator Raphael Canet said the number of people denied entry to Canada hasn't changed much since he raised the issue a week ago.
Canet says he sent two-thousand letters of inquiry to conference invitees asking about their travel plans and 70 per cent of the 315 people who replied said their visa application had been rejected by the Canadian government.
Organizers say most of the people denied entry visas are from Africa, and many are from Latin America.
Immigration Canada blame conference officials and say the rules for visitor visas need to be followed rigorously, while the event's directors said they did all they could to facilitate the visa process.
World Social Forum is an annual conference that is being held in North America for the first time.
It brings together left-leaning activists and other international civil society organizations, although most of the attendees are usually from the host nation.
