Bombers struck Shi'ite worshippers in two cities Tuesday and gunmen ambushed a busload of pilgrims in a series of attacks that killed at least 58 people as more than two million Shi'ites jammed major shrines for ceremonies marking Ashoura, the holiest day of the Shi'ite calendar. The bloodshed occurred despite heightened security following a battle with messianic Shi'ites who authorities said planned a major assault on Ashoura ceremonies. The deadliest attack occurred when a suicide bomber blew himself up among a crowd of worshippers entering a Shi'ite mosque in Mandali near the Iranian border, killing 26 people and wounding 47, according to police. At least 12 more died and 28 were wounded when a bomb exploded in a garbage can as Shi'ites were performing outdoor rituals in the largely Kurdish city of Khanaqin, police said. In Baghdad, gunmen in two cars opened fire on a bus carrying Shi'ite pilgrims to the capital's most important Shi'ite shrine, killing seven and wounding seven others, police said. Hours later, mortar shells rained down on two mostly Sunni neighborhoods, killing nine and wounding 30 in what police said appeared to be a reprisal attack. One person was killed in a mortar attack on a Shi'ite neighborhood, police said. Two policemen were killed in a boming in Mosul and one Shi'ite man was shot dead in Baghdad, police said. But intense security prevented major violence in the Shi'ite holy cities of Karbala and Najaf, venues for the biggest and most important Ashoura commemorations. Police found only eight bodies Tuesday of people slain by sectarian death squads in Baghdad, the lowest single day total in months. Ashoura ceremonies mark the 7th century death of Imam Hussein, grandson of the Prophet Mohammad, in a battle near Karbala that cemented the Sunni-Shi'ite schism. Worshippers beat themselves with chains, slice their heads with small knives and pound their chests in expressions of grief over the death of Imam Hussein. More than 1.5 million pilgrims, mostly Iraqis but from as far away as India and Pakistan, jammed Karbala for the Ashoura commemorations, according to provincial Gov. Akeel al-Khazaali. Hundreds of thousands more joined rituals in Najaf, Baghdad and other cities. In Karbala, all private transport was banned - including bicycles - and pilgrims had to submit to body-searches at dozens of checkpoints before reaching the two golden domed shrines of Imam Hussein and his half brother Imam Abbas. U.S. unmanned surveillance aircraft flew over the city to look for signs of trouble, al-Khazaali said. "Even if the terrorists tear us to pieces, we will not stop coming to visit Imam Hussein," said Abbas Karim, 27, a laborer from the southern city of Nasiriyah. Security has been tight at Ashoura commemorations since a string of bombings and suicide attacks killed at least 181 people at Shi'ite shrines in Baghdad and Karbala in 2004. Last year's Ashoura commemorations were largely peaceful, but suicide bombers killed 55 Shi'ites in 2005. This year, fears of sectarian attacks were running high because of the ongoing Sunni-Shi'ite violence, which surged after last February's bombing of a major Shi'ite shrine in the mostly Sunni city of Samarra. Security measures were further tightened after US-backed Iraqi forces fought a fierce, all-day battle last weekend with hundreds of messianic Shi'ites who officials said were planning to slaughter pilgrims and clerics during Ashoura commemorations in Najaf. Officials said more than 300 militants were killed and about 650 captured. Two US soldiers died when their helicopter crashed during the fighting. In Najaf, deputy Gov. Abdul-Hussein Abtan said that more than 300 militants were killed and 650 captured in the battle, which ended Monday. He said 11 Iraqi troops were killed and 30 wounded. With security so intense at the major venues, extremists chose targets in smaller cities where safety measures were less stringent. Both bombings occurred in Diyala province where Sunni-Shi'ite violence is raging. Khanaqin's mayor, Mohammed Mulla Hassan, said no outdoor religious events would be held in the city until further notice to avoid more bloodshed. He was referring to the anniversary of Imam Hussein's burial this Friday and ceremonies marking the 40th day after his death. Under Saddam Hussein, pilgrims from Iran were banned and even Iraq's Shi'ites, who comprise about 60 percent of Iraq's 27 million people, were restricted from performing the Ashoura rituals. After Shi'ites gained political power following his ouster, Shi'ite political parties have encouraged large turnouts as an affirmation of Shi'ite clout. That has embittered many Sunni Muslims who frown on Shi'ite rituals of self flagellation and public grief. Also Tuesday, the government announced the arrest of a provincial leader of al-Qaida in Iraq, whose late leader Abu Musab al-Zarqawi encouraged attacks on Shi'ite civilians, considering them heretics and collaborators with the Americans. Government spokesman Ali al-Dabbagh said the arrest occured in Beiji in Saddam's home province of Salahuddin. He said 59 others, including a Libyan, were arrested in a series of raids in Beiji, 250 kilometers (155 miles) north of Baghdad. Prime Minister Nouri al-Maliki said in remarks published Tuesday that he hopes sectarian militias will be dissolved and the Sunni insurgency ended within six months. Al-Maliki made the optimistic prediction in an interview with the Arabic newspaper Al-Hayat as US and Iraqi forces prepare for a security crackdown in Baghdad - the third attempt within a year to curb sectarian violence. "The militias have to end and be transferred to political organizations and any competition with the state in its attempt to bring about security must end," said al-Maliki, who owes his job in part to the backing of radical Shi'ite cleric Muqtada al-Sadr, leader of the biggest Shi'ite militia, the Mahdi Army. The US military said a Marine was killed Monday in fighting in Anbar province, an insurgent stronghold west of Baghdad, while an American soldier died in an accident northwest of Nasiriyah.