Many who arrived at the stadium Wednesday are Haitians, according to CNN's partners CTV and CBC. They exited shuttle buses holding their children and belongings as they got in line to enter the building.
In recent months, a soaring number of people have crossed from the United States to Canada. Many have expressed concerns they'll be deported under President Donald Trump's policies.
Amid the influx to Canada, shelters in Montreal are full so the Olympic Stadium is being used, CTV reported.
The stadium, which was built to host the 1976 Summer Olympics, had more than 100 green cots, along with blankets and metal dividers set up inside its massive, concrete halls.
"The city of Montreal welcomes Haitian refugees. You can count on our complete support," Montreal Mayor Denis Coderre said on Twitter.
He also tweeted that it was "yet another consequence of Donald Trump's immigration policy."
More than 4,000 people have been intercepted as they crossed into Canada this year, according to government data.
Of the 4,345 crossings, 3,350 were into Quebec.
Montreal has a large Haitian community, which may be a reason why many Haitians are crossing into Quebec, an official told the CBC
Haitians living in the United States since the 2010 earthquake
had been given temporary protected status,
which allowed them to work and shielded them from deportation. But earlier this year, Department of Homeland Security officials urged Haitian recipients to prepare for the program's potential expiration in January.
Senior DHS officials said in May that conditions in Haiti were improving since the earthquake -- and that the program could be terminated next year.
The move has sent many Haitians across the northern border.
One Haitian woman told the CBC she left the United States because she didn't "know what was going to happen."
"So we checked online and we saw that Canada was going to welcome Haitians, and that's why we come here," she told the CBC after crossing into Quebec.
In the first half of 2017, more than 18,000 people sought asylum in Canada, according to government figures.
It's already more than 75% of the total number officials registered last year.
Kathleen Weil, Quebec's minister of immigration, diversity and inclusion, said more than 6,500 asylum-seekers have come to the province from the beginning of year through June 30.
The pace of arrivals accelerated in the latter half of July, Weil said. From July 1 to July 19, an average of around 50 asylum-seekers arrived per day. But in the last two weeks, that average jumped to 150 asylum-seekers per day, she said.
Experts caution it's not so easy to meet government requirements under Canadian asylum laws and have the chance to stay.
Fearing deportation from the United States isn't enough to make an asylum case in Canada, Vilma Filici, a Toronto-based immigration consultant told CNN in May.
Just like any other country, she said, Canada has strict guidelines for granting asylum requests.
"It's not just a matter of arriving and saying, 'I'm afraid,' " she told CNN.