Iraqi security forces wearing masks and black plainclothes have opened fire on protesters in the Shiite holy city of Karbala, killing 18 people and injuring hundreds, officials say.

Key points: An eyewitness said bullets were fired towards hundreds of protesters who were sitting in an encampment

An eyewitness said bullets were fired towards hundreds of protesters who were sitting in an encampment Karbala has been gripped by a wave of anti-government protests which have often turned violent

Karbala has been gripped by a wave of anti-government protests which have often turned violent The demonstrators are fuelled by anger at corruption, economic stagnation and poor public services

The attack, which happened overnight (local time), came as Iraqis took to the streets for a fifth consecutive day, protesting against their Government's corruption, a lack of services and other grievances.

The protests, leaderless and largely spontaneous, have been met with bullets and tear gas from the first day.

At least 72 protesters — not including the latest fatalities in Karbala — have been killed since anti-government protests resumed across Iraq on Friday, after 149 were killed during an earlier wave of protests this month.

In the Karbala attack alone, more than 800 people were injured, according to one official.

Demonstrators have been protesting against government corruption and a lack of services. ( AFP )

Security officials said Tuesday's attack happened in Karbala's Education Square, about 2 kilometres from the Imam Hussein Shrine, where protesters had set up tents for their sit-in. All the officials spoke on condition of anonymity in line with regulations.

An eyewitness said hundreds of protesters were in the encampment when live bullets were fired toward them from a passing car.

Then masked gunmen in black plainclothes arrived and started shooting at the protesters, the witness said, speaking on condition of anonymity, fearing for his safety.

Tents caught fire, igniting a blaze, he added.

Demonstrators call for sweeping changes

Karbala, like Baghdad and other cities in Iraq's southern region, has been gripped by a wave of deadly anti-government protests which have often turned violent, with security forces shooting at the protesters and protesters setting fire to government buildings and headquarters of Iran-backed militias.

The demonstrations are fuelled by anger at corruption, economic stagnation and poor public services.

Despite its vast oil wealth, Iraq suffers from high unemployment and crumbling infrastructure, with frequent power outages that force many to rely on private generators.

The protests spiralled into skirmishes between demonstrators and security forces. ( AFP )

The protests have grown and demonstrators are now calling for sweeping changes, not just the Government's resignation.

Iraq's Prime Minister Adel Abdel-Mahdi has promised a Government reshuffle and a reform package, which the demonstrators have already rejected.

Authorities on Monday announced a curfew from midnight to 6:00am in the capital, as renewed protests there and across the south raged.

A senior security official estimated that 25,000 protesters took part in the demonstration in the capital.

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Thousands of students joined Iraq's anti-government protests on Monday, as clashes with security forces firing tear gas canisters killed at least three demonstrators and injured more than 100.

Students skipped classes at several universities and secondary schools in Baghdad and across Iraq's majority-Shiite south on Monday to take part in the protests, despite the Government ordering schools and universities to operate normally.

One of those killed was a 22-year-old medical student, the first woman to be killed since the protests began earlier this month. Seventeen students were among the injured.

Meanwhile, the former Iraq football captain has pleaded with the international community to step in and stop the violence.

Younis Mahmoud called on the United Nations to "intervene urgently to stop the bloodbath".

"The people of Iraq are being killed. Save Iraq," he wrote on Twitter.

Officials said more than 800 were injured during clashes in Karbala. ( AFP )

AP