(Dieu Nalio Chery ~ The Associated Press)
The unrest followed three days of similar violence in northern Haiti. The protests come a little more than a week before national elections, and the U.N. has characterized them as political. Some demonstrators threw rocks at an office of President Rene Preval's Unity party and tore down campaign posters.
But the protests are fueled by suspicions, shared by some U.S. disease experts, that a contingent of Nepalese soldiers brought cholera with them to Haiti and spread the disease from their rural base into the Artibonite River system, where the initial outbreak was centered.
The disease is new to Haiti and was not expected to strike this year despite rampant bad sanitation and poor access to drinking water.
The 12,000-member U.N. Stabilization Mission in Haiti, or MINUSTAH, which has been the dominant security force in Haiti for six years, denies responsibility for the epidemic.
Standing before the thick black smoke of blazing tires Thursday, protesters yelled "We say no to MINUSTAH and no to cholera" and carried signs reading "MINUSTAH and cholera are twins." The windows of several cars belonging to the U.N. and humanitarian groups were broken.
Haitian police fired tear gas at the protesters on the central Champ de Mars plaza, and clouds of choking irritants blew into nearby tent shelters of thousands made homeless by the Jan. 12 earthquake.
"I survived the quake but the police are going to kill me with gas," Marie Paul Moses said as she fled the white cloud.
Aid workers, including U.N. humanitarian agencies that are structurally separate from the peacekeeping force, called for calm, saying the violence is hampering efforts to treat the tens of thousands of people stricken with cholera.
The disease is spread by contaminated fecal matter. Health experts say it can be easily treated with rehydration or prevented outright by ensuring good sanitation and getting people to drink only purified water.
But after years of instability, and despite decades of development projects, many Haitians have little access to clean water, toilets or health care.
In the neighboring Dominican Republic, health authorities launched a nationwide search Thursday for people suffering from symptoms typical of cholera: diarrhea, vomiting and dehydration.
Health Minister Bautista Rojas said hundreds of doctors, epidemiologists and other medical officials will be going "house by house, in each sector, neighborhood and alleyway," looking for any trace of an outbreak.
They will interview neighbors, offer medical care and, if necessary, take anyone suspected of having cholera to the hospital.
The Dominican Republic has stepped up health measures to try to keep the epidemic from crossing the border -- especially after the nation's first cholera case was detected Monday in an immigrant brick worker who returned sick after a vacation in his Haitian homeland.
Dominican authorities have increased border patrols and monitoring of frontier crossings. The two nations share the Caribbean island of Hispaniola.