"Six demonstrators were killed and more than 20 wounded," said Mehdi al-Tamimi, head of the government's human rights council in Basra province.

Security forces "directly opened fire on protesters," he said.

Medical sources previously said two demonstrators were killed on Tuesday as thousands of people rallied outside the local government headquarters in Basra.

During the protests, some people in the crowd hurled Molotov cocktails and fireworks at the government building, while security forces responded with tear gas and by firing shots into the air.

Around 15 members of the security forces were injured in the clashes, the medical sources said.

Earlier on Tuesday, Iraqi Prime Minister Haider al-Abadi said an investigation had been launched into the death of another protester the day before.

Addressing his weekly press conference in the capital Baghdad, Abadi reasserted he had ordered "no real bullets are to be fired, in the direction of protesters or in the air".

Mekki Yasser Ashur died after being shot during protests, his family said Tuesday during his funeral march.

Protesters, some armed, accompanied his coffin through the streets of Basra until the government building before being dispersed by tear gas.

Tamimi, head of the provincial council for human rights in Basra, said security forces shot Ashur.

Protests erupted in Basra on July 8 over unemployment and the dire state of basic public services such as electricity, before spreading across southern Iraq and reaching the capital.

Iraqis have also called on authorities to clean up the water supply, with pollution blamed for putting 20,000 people in hospital in Basra province alone.

Protests had died down recently but since Friday large numbers have taken to the streets again.

Since early July, 21 people have been killed in protests across Iraq. At least one of those was shot dead by security forces, while authorities have accused "vandals" of infiltrating the rallies.