That includes O'Rourke, a potential 2020 candidate, who came close to unseating Sen.
The march is set to begin near Bowie High School, pass the convention center holding Trump's event and finish at Chalio Acosta Sports Center - a one-mile trip along the US-Mexico border.
O'Rourke told Oprah Winfrey last week he would make a final decision about running for president by the end of the month.
"People take offence at the perception and false narrative that we live at a unsafe and lawless frontier", says John Barela, a lifelong Republican who heads the Borderplex Alliance, a group promoting economic development on the US-Mexico border.
Prior to his speech, O'Rourke joined a march with thousands of people demonstrating against Trump's border wall.
"We're going to build the wall anyways", he said. "It seeks emotionally to connect with us, with voters - to stoke anxiety and paranoia, to win power over "the other" on the basis of lies that vilify people".
Even if the two sides settle the detention bed disagreement, they still remain at odds over funding for new border wall construction.
The former congressman, who is from El Paso, said that it's time to "show the country that there is nothing to be afraid of when it comes to the U.S. -Mexico border".More news: Trump sees no Xi summit by tariff date, stocking trade worries
Trump's comments about El Paso have already prompted multiple fact checks on his claims.
As for O'Rourke's rally, MSNBC reported that an estimated 7,000 people attended, while Bloomberg News reported that El Paso police estimated a crowd of between 10,000 and 15,000. "Simply put, walls work, and walls save lives".
The White House has also not ruled out another government shutdown.
On Saturday, they built a "human wall".
After Trump's remarks about El Paso in his State of the Union address, Mayor Dee Margo, a Republican, took to Twitter to counter the president's claims, saying that "El Paso was NEVER one of the MOST unsafe cities in the USA".
Trump, who said in December he would be "proud" to shut the federal government over border security, took a different tack on Monday.
The recent shutdown left more than 800,000 government workers without paychecks, forced postponement of the State of the Union address and sent Trump's poll numbers tumbling.
Trump said fencing through the city had dramatically reduced crime by separating it from the troubled Mexican city of Ciudad Juarez, just across the border. Between 1993 and 2006 the number of crimes, including home and auto robberies, dropped by more than 34 percent, according to crime statistics compiled by the Federal Bureau of Investigation, in part because of Operation Hold The Line, which deployed hundreds of agents along stretches of the border.