US agents shot several rounds of gas, according to an Associated Press reporter on the scene, after migrants attempted to penetrate several points along the border. The US Customs and Border Protection (CBP) service said that several asylum-seekers had thrown projectiles at the border patrol, which prompted the US authorities to use tear gas to disperse the crowd.
Our chief correspondent Stuart Ramsay and his team have been on the front line of the migrants' struggle to make it across.
In recent weeks, thousands of mostly Central American migrants have arrived in Tijuana by caravan.
Children screamed and coughed in the mayhem of the tear gas. Numerous migrants are seeking to apply for asylum in the USA, but as James Fredrick reported for NPR, fewer than 100 asylum petitions per day are being processed at the San Ysidro entry point.
The Trump administration has demanded that Mexico agree to host migrants who apply for asylum in the United States at the southern border as they wait for their hearing before an immigration judge in the United States.
Mexican authorities said that 39 marchers had been arrested, and that those who had "violently" and "illegally" tried to cross the US border would be deported.
Mexican police had kept them from walking over a bridge leading to the Mexican port of entry, but the migrants pushed past officers to walk across the Tijuana River below the bridge. Many hope to apply for asylum in the USA, but agents at the San Ysidro entry point are processing fewer than 100 asylum petitions a day.
US officials have said for months that they were working with Mexico to find solutions for what they have called a border crisis.
The president is putting more pressure on Mexico, warning that he might close the southern border "permanently" if necessary.
At least 500 migrants, including women and children, had been taking part in a peaceful demonstration in Tijuana before dashing toward the border in an attempt to clamber over a first metal barrier there.
Irineo Mujica, who has accompanied the migrants for weeks as part of the aid group Pueblo Sin Fronteras, said the aim of Sunday's march toward the USA border was to make the migrants' plight more visible to the governments of Mexico and the U.S.
"There is no agreement of any sort between the incoming Mexican government and the USA government", Sanchez said.
"As I have continually stated, D.H.S. will not tolerate this type of lawlessness and will not hesitate to shut down ports of entry for security and public safety reasons", Nielsen said. "Congress, fund the WALL!" he added. By Sunday afternoon, CBP reopened crossing lanes in both directions to pedestrians, CBP representative Jackie Wasiluk said.
But officials in the incoming administration of President-elect Andres Manuel Lopez Obrador said later - after the Post report was met by stiff criticism in Mexico - that the agreement was not yet final.
Meanwhile, Republican Senator Lindsey Graham of SC, took to Twitter to accuse critics of having to "really hate President Donald Trump to not understand the problems created by the caravans".
Mexico's Interior Ministry said Sunday the country has sent 11,000 Central Americans back to their countries of origin since October 19, when the first caravan entered the country.
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The president also said he's given American troops at the border the "OK" to use lethal force against migrants "if they have to". Trump said before boarding Marine One on his way to Mar-a-Lago. "Maybe he did, maybe he didn't". "It's a disgrace" he said.