US President Donald Trump has strongly defended the use of tear gas at the Mexican border to repel a crowd of migrants that included barefoot, crying children as well as angry rock-throwers.

Critics denounced the border agents’ action as overkill but the president kept to a hard line.

“They were being rushed by some very tough people and they used tear gas,” he said.

“Here’s the bottom line – nobody is coming into our country unless they come in legally.”

Mr Trump seemed to acknowledge children were affected but said it was “a very minor form of the tear gas itself” that he assured was “very safe”.

“Why is a parent running up into an area where they know the tear gas is forming and it’s going to be formed and they were running up with a child?” he asked.

Without offering evidence, he also claimed that some of the women were not really parents but instead “grabbers” who stole children so they had a better chance of being granted asylum in the US.

The showdown at the San Diego-Tijuana border crossing revealed competing narratives about the caravan of migrants hoping to apply for asylum but stuck in Mexico.

Mr Trump portrays them as a threat to US national security, intent on exploiting America’s asylum law, but others insist he is exaggerating to stoke fears and achieve his political goals.

“I think it’s so unprecedented that everyone is hanging their own fears and political agendas on the caravan,” said Andrew Selee, president of the Migration Policy Institute, a nonpartisan think tank that studies immigration.

“You can call it scary, you can call it hopeful, you can call it a sign of human misery. You can hang whatever angle you want to on it.”

Who are the migrants heading towards the US?

Migrants face security forces on the border with the US in the Mexican border town of Tijuana. Photo: Getty

Mr Trump rails against migrant caravans as dangerous groups of mostly single men.

That view featured heavily in his speeches during the mid-term election campaign when several were hundreds of kilometres away, travelling on foot.

Officials said about 500 members are criminals but did not back that claim up.

Mr Trump tweeted the caravan at the border included “stone-cold criminals”.

At the sports complex where most of the migrants in the caravan are staying, there are 4938 people there.

That includes 933 women, 889 children and 3105 men, Tijuana’s social services department director Mario Figueroa said.

A child looks skyward as migrants in Tijuana demonstrate at the US border. Photo: Getty

About 300 troops who had been deployed in south Texas and Arizona as part of a border security mission were moved to California.

The military’s role is limited largely to erecting barriers along the border and providing transportation and logistical support to Customs and Border Protection.

Democrats and immigrant rights groups blasted the border agents’ tactics.

“These children are barefoot. In diapers. Choking on tear gas,” California Governor elect Gavin Newsom tweeted.

“Women and children who left their lives behind – seeking peace and asylum – were met with violence and fear. That’s not my America.”

US Army soldiers fortify the US-Mexico border wall with barbed wire at Mexicali, Mexico. Photo: Getty

The day after the incident, Mexico’s foreign ministry asked the US for “a full investigation”.

US Customs and Border Protection commissioner Kevin McAleenan said the administration’s concerns about the caravan “were borne out and on full display” on Sunday.

Mr McAleenan said hundreds – perhaps more than 1000 – people attempted to rush vehicle lanes at the San Ysidro crossing.

Mexican authorities estimated the crowd at 500.

The chaos followed what began as a peaceful march to appeal for the US to speed up processing of asylum claims.

–ABC/agencies