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

During the procession some armed civilians fired shots into the air and hailed Ashur as a "martyr".

- Promises from Baghdad -

Police and military cordons had been put in place, blocking numerous roads, while Basra's hospitals were filled with protesters bringing in people wounded in the clashes.

Basra's human rights council chief said tension was high in the city and "the shops have shut a lot earlier than usual".

Tamimi warned of further escalation "if the government's doesn't respond to the demands of demonstrators".

The government has already announced an emergency plan in response to the protests, while promising billions of dollars in investment for southern Iraq.

But protesters are wary of such pledges from the outgoing administration, as Iraqi lawmakers are in the process of forming a new government after May's election.

Political corruption has also fuelled protests, which erupted in Basra on July 8 before spreading across southern Iraq and reaching the capital.

A litany of social problems -- from unemployment to electricity cuts -- have also been a central complaint of demonstrators.

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.