(AP) -- Masked gunmen opened fire at Iraqi protesters in the Shiite holy city of Karbala on Tuesday, killing 18 people and wounding hundreds, security officials said, in one of the deadliest single attacks on protesters since anti-government demonstrations erupted earlier this month.
The attack, which happened overnight, came as Iraqis took to the streets for a fifth consecutive day, protesting their government's corruption, lack of services and other grievances.
The bloodshed in Karbala, a major pilgrimage site where a revered Shiite figure was killed in a 7th century battle, could mark a turning point in the demonstrations.
It was not immediately clear who was behind the attack, and protesters said they did not know whether the masked men were riot police, special forces or Iran-linked militias. The protesters said Iraqi soldiers had been stationed around the protest site but withdrew after the attackers began firing tear gas and live fire.
The provincial governor, Nassif al-Khutabi, denied that any protesters were killed, adding there were some injuries among security forces.
The protests across Iraq, leaderless and largely spontaneous, have been met with bullets and tear gas by security forces from the first day.
At least 73 protesters — not including the latest fatalities in Karbala — have been killed since anti-government demonstrations resumed on Friday, after 149 were killed during an earlier wave of protests this month.
Security officials said Tuesday's attack happened in Karbala's Education Square, about 2 kilometers (1.2 miles) 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.
Karbala, along with Baghdad and cities across southern Iraq, has been gripped by a wave of anti-government protests which have often turned violent, with security forces opening fire and protesters torching government buildings and headquarters of Iran-backed militias.
The demonstrations have erupted in Shiite-majority areas and have been directed at the Shiite-dominated government and Shiite political parties and militias, many of which are supported by neighboring Iran.
Karbala is one of the holiest places in Shiite Islam, where Hussein, the grandson of the Prophet Muhammad, was killed in battle in A.D. 680, a formative event in the schism between Shiite and Sunni Muslims. Hundreds of thousands of Shiites flock to the city every year to commemorate his martyrdom.
The overwhelming majority of the residents in Karbala and the city's security forces are Shiite.
The demonstrations are fueled 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 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 a.m. 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.
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 wounded 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 female medical student, the first woman to be killed since the protests began earlier this month. Seventeen students were among the wounded.