BAGHDAD, Iraq (CNN) -- Gunmen on Wednesday targeted 27 Baghdad mosques and killed three Sunni imams in the wake of a bomb attack at one of the holiest of Shiite sites.

The wave of attacks followed a bombing at the Al-Askariya "Golden Mosque" in Samarra. The strikes, involving small arms, rocket-propelled grenades and mortar rounds, all happened between 11 a.m. and 5 p.m., police told CNN.

Three of the mosques were burned down, and in addition to the imams, three mosque guards were also killed. One imam was kidnapped.

Kurdish, Shiite and Sunni leaders who have been working to form a national unity government urged citizens to remain calm and united, amid fears that sectarian violence would escalate into civil war.

Meanwhile, the top half of the golden dome that once towered above the Al-Askariya "Golden Mosque" lay collapsed in the blast. Minarets flanking the cupola remained standing, amid mounds of debris. (Watch how the blast is intensifying tension -- 1:51)

The Samarra attack happened at 7 a.m., when gunmen dressed as Iraqi police commandos bombed the site, which has deep significance in early Islam. Salaheddin province is home to a large Sunni Arab population.

There were no immediate reports of injuries.

Foreign fighters were likely responsible, and the attack bore the hallmarks of al Qaeda in Iraq, Iraqi National Security Adviser Mowaffaq al-Rubaie said.

"They are really testing the patience of the Iraqi people," he said, calling on Muslims around the world to condemn "this act of terrorism."

Ten people -- all dressed as Iraqi police commandos -- have been arrested, al-Rubaie said.

Both Iraqi Prime Minister Ibrahim al-Jaafari, a Shiite, and President Jalal Talabani, a Kurd, said the attack was an effort to incite sectarian violence.

In a taped address on Iraqiya TV, al-Jaafari called on all Iraqi political parties to condemn the mosque attack and asked Sunni and Shiite Iraqis to demonstrate in Samarra, about 70 miles (113 kilometers) north of Baghdad.

Al-Jaafari also declared a three-day period of mourning and ordered the Ministries of Defense and of Housing and Reconstruction to assess damage and begin quickly rebuilding the shrine.

Talabani condemned the "horrendous crime," saying, "The perpetrators' aim is to start a civil war between Iraqis."

U.S. Ambassador to Iraq Zalmay Khalilzad called the attack a "critical moment" for Iraq and a "crime against humanity." Reading a joint statement from Gen. George Casey, the top-ranking military leader in Iraq, Khalilzad pledged that the United States will help in the mosque's reconstruction.

"This desperate and despicable act shows that terrorists stop at nothing and care for nothing," Khalilzad said. "May God bless the people of Iraq."

The Iraqi Islamic Party, Iraq's largest Sunni Arab movement, called the attack "an extreme crime" and called for an investigation into who was responsible. Later, its southern Baghdad office and three local Sunni mosques were attacked, police said.

Gunmen stormed the Iraqi Islamic Party's office, evacuated its employees and torched the place. The main entrance to Al-HamzaMosque, which escaped damage, came under machine-gun fire and was burned. A rocket-propelled grenade caused damage to the entrance of the Al-Mustafa Mosque. And gunmen stormed the Ali Al-Athim Mosque and burned it.

Security was being beefed up around all mosques in Baghdad, police said.

Peaceful protests, not violence, urged

Following noon prayers, 4,000 demonstrators in Baghdad walked from the office of Shiite cleric Muqtada al-Sadr toward a nearby mosque, a spokesman for his office said. By the time the march ended, the number of people had swelled to 10,000, the spokesperson said. It was one of several protests in Baghdad neighborhoods, Iraqi Emergency Police said.

Al-Sadr cut short a visit to Lebanon and was returning to Iraq, where he was planning to make a televised statement.

A senior spokesman for al-Sadr's movement urged "calm and patience" and said maintaining national unity is Iraq's top priority.

The Grand Ayatollah Ali al-Sistani is calling for a week of mourning, said Hamed al-Khafaf, a spokesman for the powerful Shiite cleric.

Al-Sistani is expected to call for all businesses to close in protest for three days and for all Shiite and Sunni Iraqis to express themselves in peaceful protest, not with violence, al-Khafaf said.

In Samarra, several hundred people gathered at the mosque and at the mayor's office, denouncing the Iraqi government and the U.S. military, authorities said. Thousands of protesters also took to the streets in Najaf, Kut and Karbala, police said.

The Al-Askariya Golden Mosque is important to Shiites, because they believe Imam al-Mehdi, the 12th and final awaited imam, will appear at the mosque to bring them salvation.

He is the son of Imam Hassan al-Askari, the 11th imam, buried in the shrine. His grandfather, the 10th imam, is also buried there.

Al-Mehdi is said to have disappeared in the eighth century during the funeral of his father and is believed by Shiites to have been withdrawn by God from the eyes of the people. They are waiting for him to reappear as their leader, believing the event will take place in Samarra.