LOS ANGELES — Drawing on an American history of cruelty, from the conquest of the Indians to the slave trade to the internment of Japanese-Americans during World War II, activists in this city gathered on Thursday to voice outrage at what they see as the latest affront to American values by the Trump administration: splitting up migrant families at border crossings, and confining children in detention facilities.

“I think we are fighting for the heart and soul of America,” Yolanda Varela Gonzalez, a teacher and activist, told a crowd of several hundred protesters in MacArthur Park, before they marched to an immigrant detention center in downtown Los Angeles.

From the start, the Trump administration’s hard line on immigration has galvanized this city of immigrants, where nearly half the population is Latino, and where deportations and roundups of undocumented immigrants by government agents has spread fear and anger in immigrant neighborhoods. And the stories they have heard lately, and the images they have seen, of children being ripped from the arms of parents at the border have heightened a sense of outrage, a sense that they are experiencing an era of American history that will be looked back upon with shame.

“Taking kids from their parents is crossing a line,” said Gale Chernich, who joined the protest in Los Angeles.