Tijuana, Mexico (CNN) The man held a backpack in one hand and a folder in the other.

Reporters swarmed around him as Mexican immigration officials whisked him into a van and drove away.

The chaotic scene of one Honduran migrant stepping foot on Mexican soil Tuesday marked the beginning of a major change in US policy.

More than a month after Homeland Security Secretary Kirstjen Nielsen announced the United States would force some asylum seekers to remain in Mexico while their cases make their way through US immigration courts, and days after officials revealed the policy would begin, the Trump administration's new measure apparently began with one migrant returned back over the border.

It was unclear how many others would follow.

Rodulfo Figueroa, a representative from Mexico's National Migration Institute, told reporters after the man's arrival that the United States wouldn't be sending any more migrants to Mexico on Tuesday. A Department of Homeland Security official confirmed that one person had been returned to Mexico under the new policy, and that others would be processed Wednesday and for the remainder of the week.

The policy's official debut at the San Ysidro Port of Entry came as Nielsen visited the port.

On Tuesday morning, a CNN team in Tijuana observed a group of about 35 migrants loaded onto official vehicles bound for the United States. Hours later, a single man returned to the Tijuana side. It wasn't clear how long he'd been in the United States.

Figueroa said that man carried a Mexican humanitarian visa and had an appointment to return to the United States to continue the asylum process. The man was part of a migrant caravan that entered Mexico in November, Figueroa said. His name was not available.

Administration officials say the new policy "will help restore a safe and orderly immigration process, decrease the number of those taking advantage of the immigration system, and the ability of smugglers and traffickers to prey on vulnerable populations, and reduce threats to life, national security, and public safety, while ensuring that vulnerable populations receive the protections they need."

It's expected to face legal challenges.

Immigrant advocacy groups have been sharply critical of the new policy since it was announced in December, and they doubled down last week as word spread that officials were going to start putting it into practice. They say the measures, dubbed "Migrant Protection Protocols" by the administration, are quite the opposite, arguing that forcing asylum seekers to wait in Mexico while their cases make their way through US courts puts vulnerable people in even more dangerous situations.

The head of the Border Patrol union has also criticized the policy, saying it will incentivize illegal immigration and create more stress for Border Patrol agents.

A number of key questions about the policy remain unanswered. Chief among them: How will it actually play out on the ground?

According to Mexico's state-run Notimex news agency, the national coordinator of Civil Protection said Tuesday that Mexico is "not prepared either legally or operationally" to receive Central American migrants returning from the United States.

Mexican officials have maintained they had nothing to do with the policy, calling it a unilateral decision by the United States. But they've also said that for humanitarian reasons they will accept returning asylum seekers, though they've laid out some limits, stressing for example that Mexico will not accept the return of any unaccompanied minors or people with health problems.

Human and immigrant rights groups in Mexico have accused Mexican authorities of being complicit.

"The US Government violates asylum seekers' rights and fails to uphold responsibilities under international law when it expels them to Mexico while they seek refuge in the US. Likewise, the Mexican government is accomplice to this violation when it agrees to accept asylum seekers who fled their countries through Mexico to seek refuge in the US," the Pueblo Sin Fronteras advocacy group said in a statement Friday. "The US government wants to turn Mexico into a vast immigration detention center."

The new measure's implementation comes at a time when a large number of migrants have been waiting for weeks for the opportunity to seek asylum at the US port, where officials generally process a relatively small number of cases daily.

Hours after the first group had crossed Tuesday morning, another group of about 45 asylum seekers was still waiting on the Mexican side of the border to be processed Tuesday afternoon.