In all corners of Puerto Rico, from the central mountains to the southern coast, the protesters set their alarms for early on Monday morning — in some cases, so early that it was still dark.

They grabbed flags and whistles, sunscreen and umbrellas, and wound their way into the capital. By the time they reached Expreso Las Américas, a major highway, many more fellow protesters awaited with catchy chants and folk music, ready to shut down the island in a political show of force against Gov. Ricardo A. Rosselló.