Protesters blocked the entrance to Iraq's Umm Qasr commodities port and set fire to the main provincial government building where they had been demonstrating for a third night.
At least three protesters died during violent unrest in Iraq's Basra on Thursday, local health and security sources said.
The protesters died during attacks on local government and political party buildings, the sources said, bringing the total number of dead protesters since widespread demonstrations kicked off on Monday to 10.
At least 10 members of the security forces were also wounded, the sources said.
Southern Iraq, heartland of the Shia majority, has erupted in unrest in recent weeks as protesters express rage over collapsing infrastructure, power cuts and corruption.
Residents in Basra, a city of more than two million people, say the water supply has become contaminated with salt, making them vulnerable and desperate in the hot summer months. Hundreds of people have been hospitalised from drinking it.
Overnight, protesters blocked the entrance to the nearby Umm Qasr port, the main lifeline for grain and other commodity imports that feed the country.
They blocked the highway from Basra to Baghdad and set fire to the main provincial government building where they had been demonstrating for a third night.
Protesters continued on Thursday to block the entrance to the port, port employees and local officials said, but it was not yet possible to determine whether the unrest would have a serious impact on its operations.
Umm Qasr receives grain, vegetable oil and sugar shipments that feed a country largely dependent on imported food.
Public anger has grown at a time when politicians are struggling to form a new government after an inconclusive parliamentary election in May. Residents of the south complain of decades of neglect in the region that produces the bulk of Iraq's oil wealth.
Earlier on Wednesday, security forces sprayed tear gas and fired into the air to try to disperse demonstrators. According to health sources, the dead protester was struck in the head by a smoke grenade during the clashes.
The deaths of five protesters in clashes with security forces on Tuesday added to the fury. Security and health sources said 22 members of the security forces had been injured in Tuesday's violence, some by a hand grenade.