At least three Iraqi protesters have been killed during an attack on the Iranian consulate in the Shia holy city of Karbala, according to security sources and a monitor.

Demonstrators reportedly scaled the concrete barriers surrounding the building late on Sunday. They hung Iraqi flags and spray-painted "Karbala is free, Iran out, out!" on them, in a sign of increasing anger among some demonstrators against Tehran's perceived interference in Iraqi politics, according to Al Jazeera.

Security forces fired in the air to disperse the crowd, who threw stones and burned tires around the building on a street corner in Karbala, south of the capital, Baghdad.

As the crowds grew, heavy gunfire and volleys of tear gas rang out.

Mustafa Saadoon, director of the Iraqi Observatory for Human Rights, told Al Jazeera on Monday that three protesters had been killed overnight. Security and medical sources quoted by news agencies gave the same death toll.  

The Associated Press says the protests are directed at a postwar political system and a class of elite leaders that Iraqis accuse of pillaging the country's wealth while the country grows poorer. But protesters have also directed their rage at neighboring Iran and the powerful Iraqi Shiite militias tied to it.

The anti-government protests in Karbala, Baghdad and cities across southern Iraq have often turned violent, with security forces opening fire and protesters torching government buildings and headquarters of Iran-backed militias. More than 250 people have been killed in the security crackdown this month.

Iran has urged protesters to seek changes within the "framework of legal structures", warning them that "the enemy wants to break" the structures.

Iran’s Press TV says Iraq’s Ministry of Foreign Affairs has condemned an attack on the Iranian consulate in Karbala, reaffirming Baghdad’s commitment to protection of diplomatic missions in Iraq.  

The ministry censured on Monday the attack by a group of so-called demonstrators on the Iranian consulate and voiced Baghdad’s commitment to the security and "inviolability" of foreign missions in the country, according to Press TV.  .

“The Ministry of Foreign Affairs condemns the attack by some demonstrators on the Consulate General of the Islamic Republic of Iran in the holy city of Karbala, and reaffirms the ministry’s commitment to the inviolability of diplomatic missions guaranteed by the Vienna Convention on Diplomatic Relations and the need not to endanger their security,” the statement said.