The protest, organised by local authorities and Mexican advocacy groups, brought together people armed with flowers, including politicians, social leaders and crowds of students to the border town Ciudad Juarez - which already is separated by extensive fencing from its American neighbor city El Paso.
Protesters hurled slogans at Trump, whose plans to build the wall to keep undocumented immigrants out of the US - and make Mexico foot the bill - has enraged many people here.
"The wall is one of the worst ideas," said Carolina Solis, a 31-year-old student. "It won't stop anything - not drugs or migrants."
"It's just a symbol of Donald Trump's hatred, the president's racism."
Under the watchful eye of US Border Patrol officers, protesters - among them El Paso Mayor Oscar Leeser - formed a human barrier of nearly 1.5 kilometres.
Young people wave colored flags reading "Peace" as they form a symbolic human wall along the Rio Grande. (AAP) Source: AAP
Many people on both sides of the border cross it daily, calling one country home while going to work in the other.
"Ciudad Juarez and El Paso are one city - we will never be apart," said Leeser, who was born on the Mexican side of the border.
His Ciudad Juarez counterpart Mayor Armando Cabada vowed to help resettle migrants deported from the US.
"Trump only generates fear in our US compatriots. We must show solidarity with them and tell them that they have our support," he said.
"If they are deported, we will welcome them with open arms."
Last week, Immigration and Customs Enforcement (ICE) agents arrested some 680 people across the United States as part of a crackdown by the new administration on the estimated 11 million undocumented immigrants living in the United States.
Officials insist the raids targeted known criminals but rights advocates say people with no serious criminal records were also detained.
A similar protest was planned on Mexico's Pacific coast, at the border between the city of Tijuana and its US neighbor San Diego.
