U.S. border agents sprayed tear gas at migrants attempting to cross the border.

The Central American migrants were going through a border crossing between Tijuana and California, according to an Associated Press (AP) reporter at the scene.

The incident happened shortly after the U.S. suspended pedestrian crossings at the San Ysidro port of entry, one of the busiest entry points between the U.S. and Mexico.

Children were screaming and coughing in the mayhem.

Honduran migrant Ana Zuniga, 23, said she saw migrants open a small hole in concertina wire at a gap on the Mexican side of a levee, at which point US agents fired tear gas at them.

"We ran but when you run the gas asphyxiates you more," she told the AP while cradling her three-year-old daughter Valery in her arms.

US Border Patrol helicopters flew overhead, while agents held vigil on foot beyond the wire fence in California.

Earlier today, several hundred Central American migrants pushed past a blockade of Mexican police who were standing guard near the international border crossing.

They appeared to easily pass through without using violence and some of the migrants called on each other to remain peaceful.

A second line of Mexican police carrying plastic riot shields stood guard outside a Mexican customs and immigration plaza.

That line of police had installed tall steel panels behind them outside the Chaparral crossing on the Mexican side of the border.

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 the caravan.

Most of the migrants are fleeing rampant violence and poverty in their home countries.

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.

President Donald Trump earlier today tweeted: "Would be very SMART if Mexico would stop the Caravans long before they get to our Southern Border, or if originating countries would not let them form (it is a way they get certain people out of their country and dump in U.S. No longer). Dems created this problem. No crossings!"