“We must always remember our past, but that does not mean we must let it define us,” Mr. Gunn said in a statement that stunned many in Jackson, the capital, and was seen as adding a highly fraught issue with statewide elections there this year. “As a Christian, I believe our state’s flag has become a point of offense that needs to be removed,” Mr. Gunn said.

For decades, images of the Confederacy have been opposed by people who viewed them as painful symbols of slavery, racism and white dominance, and supported by those who saw them as historical emblems from the Civil War, reminders of generations-long Southern pride. Yet the new calls, after the church massacre last week, came with surprising force and swiftness. The demands straddled lines of partisanship and race, drawing support even from Southern conservatives who for years had defended public displays of the flag as a matter of regional pride. The movement also reached far beyond the political sphere, and beyond the South itself.

In Minnesota, activists demanded that a lake named after John C. Calhoun, a senator and vice president from South Carolina who was a proponent of slavery, be renamed. Amazon and eBay announced on Tuesday that they would no longer allow the sale of Confederate flags and similarly themed merchandise, joining Walmart and Sears, which had already done so. And messages were painted on Confederate statues in Charleston; Baltimore; and Austin, Tex., that read: “Black Lives Matter.”

“To see all of this happening, all of a sudden, it speaks of some fundamental change in the country,” said Kerry L. Haynie, a political scientist at Duke University. “It is surprising in the sense that there have been calls for this for years. But it took this tragedy to spur this type of change.”