The "Remain in Mexico" policy, which is expected to face a legal challenge, is to be introduced at the busy Tijuana-San Ysidro border area.
At least three volleys are sent into a crowd of about 150 trying to breach the border fence near Tijuana's beach in the early hours of 2019.
Agents in riot gear stop protesters as they try to move forward to offer a ceremonial blessing near the wall in support of asylum seekers.
The future interior chief says there is no agreement with the Trump administration on a "Remain in Mexico" plan for Central American migrants.
City officials and volunteers in the border town are assisting nearly 5,000 migrants who have arrived after more than a month on the road.
Tensions build as nearly 3,000 people seeking asylum in the U.S. pour into the Mexican city after weeks on the road.
The Mexican border city has converted a municipal gymnasium into a temporary shelter for the newcomers.
Because of a hostile Trump administration, even asylum seekers who establish "credible fear" could face years of detention.
Hundreds of Central Americans rally at the border between Tijuana, Mexico, and San Diego as they prepare to directly challenge the Trump administration's immigration policies.