“The air is thick, thick, thick with tear gas,” said Amar Mohamed, 25, a taxi driver who was marching in Khartoum North, a locus of the protests a short drive across the Blue Nile river from downtown Khartoum, the country’s capital. On a phone call, he described a chaotic scene with protesters scattering into alleyways. “They are trying to pick us up and put us into their trucks.”