SAN ANTONIO — For months, a migrant-services center blocks from the Alamo in downtown San Antonio has been packed with Central American families who have crossed the border in record-breaking numbers.

But in recent days, hundreds of migrants from another part of the world have caused city officials already busy with one immigrant surge to scramble on a new and unexpected one. Men, women and children from central Africa — mostly from the Democratic Republic of Congo and Angola — are showing up at the United States’ southwest border after embarking on a dangerous, monthslong journey.

Their arrival at the border and at two cities more than 2,100 miles apart — San Antonio and Portland, Me. — has surprised and puzzled immigration authorities and overwhelmed local officials and nonprofit groups. The surge has prompted Portland to turn its basketball arena into an emergency shelter and depleted assistance funds meant for other groups. Officials in both cities have had to reassure the public that fears of an Ebola outbreak were unfounded while also pleading for volunteer interpreters who speak French and Portuguese.

In San Antonio, the city-run Migrant Resource Center has assisted about 300 African migrants who were apprehended at the border and released by the authorities since June 4. Those 300 are just a portion of the overall numbers. Since October 2018, more than 700 migrants from Africa have been apprehended at what has become their main point of entry, the Border Patrol’s Del Rio sector, a largely rural stretch of Texas border that is nearly 200 miles west of San Antonio.