Donald Trump has demanded that Mexico move thousands of migrants, gathered at the US border, back to their home countries.
He vowed they will never enter America, and threatened to close the border permanently if necessary, to tackle the growing crisis.
It came after some migrants tried to breach the crossing point in Tijuana, yesterday, where around 9,000 people are camped.
US agents fired tear gas into the crowd, which included children, as a peaceful march turned into chaos.
Children were screaming and coughing in the mayhem.
Mexico is looking set to shore up security near the border too, as police lined up outside a shelter in Tijuana and told Central American migrants they couldn't walk toward the border area.
Mexico's National Migration Institute said that 98 migrants were being deported after the attempted breach.
Mexican officials said the migrants had taken part in "violent" chaos, which originally began as a peaceful march.
On Monday, pedestrian crossings were suspended at the San Ysidro port of entry at both the east and west facilities, and all northbound and southbound traffic was halted.
US Homeland Security Secretary Kirstjen Nielsen said in a statement that US authorities would continue to have a “robust” presence along the border and that they would prosecute anyone who damaged federal property or violated US sovereignty.
“DHS (Department of Homeland Security) will not tolerate this type of lawlessness and will not hesitate to shut down ports of entry for security and public safety reasons,” she said.
More than 5,000 migrants have been camped in and around a sports complex in Tijuana after making their way through Mexico in recent weeks via caravan.
Many hope to apply for asylum in the US but agents at the San Ysidro entry point are processing fewer than 100 asylum petitions a day.
Tijuana Mayor Juan Manuel Gastelum on Friday declared a humanitarian crisis in his border city of 1.6 million, which he says is struggling to accommodate the crush of migrants.