Thousands of migrants began arriving in Tijuana about 10 days ago and have been housed since then in squalid conditions in a community sports center that has been converted into a makeshift shelter.
US border agents shot several rounds of tear gas after some migrants attempted to penetrate various points along the border and threw what appeared to be rocks at USA authorities.
The gas reached hundreds of migrants protesting near the border after some of them attempted to get through the fencing and wire separating the two countries.
"We ran", a 23-year old Honduran woman told the Associated Press, "but when you run the gas asphyxiates you more".
Fani Caballero, 32, a migrant from Honduras, sat by the train tracks, within sight of USA agents on the other side of the border fence.
Customs and Border Protection tweeted about the alleged use of tear gas.
Needless to say, it's an intense scene.
More demonstrations were planned on Sunday at San Ysidro, the busiest land port of entry in the Western Hemisphere, according to the US General Services Administration. "The vast majority of people we're dealing with are adult males".
Trump has deployed military forces to the border to support the Border Patrol and threatened on Saturday to close the entire southern border.
Stephanie Leutert, director of the Mexico Security Initiative at the University of Texas at Austin, described the Remain in Mexico plan as a strategy to take away the ability of migrants to live and work in the USA while cases are processed.
Videos posted on Twitter showed large numbers of migrants dashing across a shallow concrete waterway towards the border.
President Trump offered Mexico some advice on deporting migrants massed in Tijuana and threatening to storm the US, telling the neighboring nation in a tweet to send them "back to their countries" by plane or by bus - or face a permanent closure of the border.
"We can't have all these people here", Mujica said. "Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the United States of America", the president said in a tweet. "Congress, fund the WALL!" the United States president tweeted on Monday.
Trump has repeatedly suggested without evidence that the migrant caravans are full of hardened criminals, but they are mostly poor people with few belongings who are fleeing gang violence. Many residents of Tijuana work, study and visit the United States each day, and the border closures affect them, too, Juan Manuel Gastlum said on Twitter on Sunday. During his presidential campaign, he promised he'd have Mexico pay for the wall. If they agreed to the "Remain in Mexico" policy, those caravan members - and nearly certainly many thousands more who are on their way to the USA - would have to be cared for in Mexico while they await the outcome of their asylum requests. News slowly spread as the Thanksgiving holiday weekend wrapped up and millions were traveling back home, but by Monday morning countless Americans expressed outrage.