TIJUANA, Mexico (AP) — Central Americans who traveled in a caravan through Mexico to the border with San Diego have begun turning themselves in to U.S. authorities to seek asylum in a challenge to the Trump administration.

The migrants began applying for protection at the nation’s busiest border crossing Monday after immigration officials said the facility didn’t have space to accommodate the group.

U.S. Customs and Border Protection said didn’t say how many were allowed in late Monday but project organizer Alex Mensing of Pueblo Sin Fronteras said there were eight.

It marked the end of a monthlong journey by foot, freight train and bus for the migrants, many of whom are fleeing violence in their home countries.

The caravan got attention after President Donald Trump and members of his Cabinet called it a threat to the United States.

President Trump says a caravan of Central American migrants seeking asylum in the U.S. shows the weakness of the nation’s immigration laws.

Reacting to news that the group reached the U.S. border over the weekend, Trump tweeted Monday that the group is “openly defying our border.”

He called out Democrats for supporting “sanctuary city” policies that limit local police cooperation with federal immigration authorities.

Trump and other members of his administration have been watching and criticizing the caravan of hundreds of Central Americans since they started journeying through Mexico a month ago.

The migrants tried to start applying for protection at the San Diego border crossing Sunday.

But U.S. officials have said for two days that the facility is full and can’t accommodate them.

© Copyright 2018 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.