A massive migrant caravan organized via QR code left the town of Huehuetán in the south of Mexico Monday morning on the third day of their long trek toward the U.S. southern border.The caravan, made up of mostly Central Americans, South Americans and Haitians, is the largest and most organized of its kind this year, with participants registering to join via QR code starting on Oct. 15.TENS OF THOUSANDS OF MIGRANTS GATHERING AT SOUTHERN BORDER AHEAD OF ‘REMAIN IN MEXICO RESTART’ SOURCES SAY “Tell Biden we are coming,” one migrant named William from El Salvador told Fox News.Video footage captured by Fox News showed thousands of migrants, including small children being pushed in strollers, walking north about 20 miles north of Tapachula. One migrant carried a large wooden cross at the front of the caravan while others carried American flags and signs with President Biden’s name.”President Biden, we need your help,” one migrant said.Unlike the caravans that made headlines in 2019, this one will take considerably longer. The Mexican government has banned truckers from letting migrants hitch a ride, so they will have to walk the entire 2,500 miles to the Texas border.Mexican officials told Fox News that there are more than 3,000 migrants in the caravan, which left Tapachula on the border with Guatemala on Saturday.Within hours of their trek, the flood of migrants encountered a wall of Mexican interior police wearing protective gear and wielding shields. But the security did not hold, as migrants forced their way through the blockade and continued on their way to try and seek entry into the U.S.Customs and Border Protection announced Friday that there had been more than 1.7 million migrant encounters in fiscal year 2021, breaking records. Additionally, there were more than 192,000 encounters in September alone – a slight decrease from the more than 200,000 encountered in July and August, but a sign that the border crisis that has dogged the Biden administration is far from over. The administration also had to deal with a massive Haitian migrant surge in September, and fears that more surges could be on the way.The Biden administration has blamed root causes like poverty, corruption and violence in Central America elsewhere and has pledged massive investments of taxpayer money to try and combat those root causes.Republicans, meanwhile, have pointed to the rapid rollback of Trump-era border policies by the Biden administration – particularly the “Remain in Mexico” policy that required asylum seekers to stay in Mexico while they await hearings on their requests for safe haven in the U.S. The Biden administration has vowed to try to terminate the Remain in Mexico policy again through a different method after a federal court in Texas ruled that the administration’s previous memorandum on ending the policy was against the law.CLICK HERE TO GET THE FOX NEWS APPThe Department of Homeland Security said it will reinstate the policy by mid-November in response to the court order upheld by the Supreme Court. Border Patrol sources told Fox News that as many as 60,000 migrants are amassing on the Mexican side of the border and intend to enter the U.S. in the coming days in anticipation of the policy’s re-implementation.Fox News’ Adam Shaw contributed to this report.