BAGHDAD (CNN) -- A dramatic and bloody outbreak of violence in northern Iraq Tuesday stoked even more agitation in a country simmering with Sunni-Shiite tensions.

In northern Iraq, at least 28 people were killed and more than 70 wounded in clashes between security forces and gunmen in Hawija, in Kirkuk province, according to a senior Iraqi army officer who spoke on condition of anonymity because he is not authorized to speak to the media.

Police said the fighting began when security forces looking for wanted people raided al-Atisam Square, the site of ongoing demonstrations by Sunni Muslims.

But Sunni politicians claim peaceful Sunni protesters were assaulted during the raid and blame Nuri al-Maliki, the Shiite prime minister.

Iraqi House Speaker Osama al-Najafi issued a strong condemnation, saying the army opened fire on peaceful protesters and called the military "a tool to suppress the people and not to defend them."

"We condemn in the strongest words of condemnation and denunciation the unfortunate crime committed by the army against the demonstrators in Hawija," he said.

Al-Maliki ordered an investigation, aid to the wounded, and compensation to their families.

Two Sunni Cabinet members -- Minister of Education Mohammed Tameem and Minister of Science and Technology Abdul Karim al-Samarrai -- have submitted their resignations in protest over the actions by security forces, their offices said.

Martin Kobler, the U.N. special representative for Iraq, was in Kirkuk to meet with local officials and discuss the Hawija violence, state TV reported.

The situation is now very tense in provinces with large Sunni populations -- such as Anbar, Salaheddin, Nineveh and Diyala, according to a senior Interior Ministry official in Baghdad, who also spoke under condition of anonymity.

Tensions boiled over Tuesday in the Anbar province city of Falluja. Citizens who were angered by the Hawija events set two army vehicles on fire, and others near Falluja attacked an army convoy.

Al-Najafi urged security forces and Sunni tribes battling across the country to restrain themselves, warning of a civil war if such fighting continues.

Sunni-Shiite tensions

Sunnis, who comprise a minority of Iraqis, had clout during the Saddam Hussein era, but have been politically marginalized since his overthrow. Shiites, who make up a majority of Iraqis, dominate the government.

Since December, tens of thousands of demonstrators have taken to the streets of predominately Sunni provinces -- including Anbar, Nineveh, Salaheddin and Diyala -- demanding that the Shiite-led government stop what they call second-class treatment of Iraq's Sunni community.

Such turnouts include protests at al-Atisam Square in Hawija.

Over the past few months, al Qaeda in Iraq has claimed responsibility for several attacks across the country that targeted Shiite areas and Iraqi security forces.

A number of attacks have targeted the Sunni community in different locations over the past week. The deadliest one was at a popular coffee shop in western Baghdad, which left at least 27 people dead and 51 others wounded.

Earlier Tuesday, another attack occurred. At least four people were killed and 13 others were wounded when two roadside bombs exploded, one after another, outside a Sunni mosque in southern Baghdad, police said. The incident occurred in the neighborhood of Dora while Sunni worshipers were leaving the mosque, police said.

There was some violence during Saturday's Iraqi provincial elections, but there weren't major incidents and the polling was considered a success by Iraq, the United States, and the United Nations.

CNN's Joe Sterling contributed to this report