WATCH: A crowd estimated in the millions flooded Karbala on December 27 to mark Ashura. To show solidarity and remorse for not being able to save Imam Hussein, some mourners self-flagellate or hit their heads with swords. Pilgrims dressed in black took part in a ritual 5-kilometer run, known as the "Twairij," to Imam Hussein's shrine.
KARBALA, Iraq (Reuters) -- Millions of Shi'ite Muslims were at shrines and mosques across Iraq for the Ashura religious festival
while Iraqi forces stood watch against the bloody attacks that have marred past pilgrimages.
Loudspeakers blared traditional Ashura chants across Baghdad and the city of Karbala, site of the most important shrine where Shi'ites commemorate the slaying of the Prophet Muhammad's grandson Hussein in 680 AD.
Saddam Hussein, a Sunni, placed strict limits on the traditional pilgrimage to Karbala, but since his overthrow in 2003 Ashura has become a show of strength for Iraq's Shi'ite majority and a prime target of Sunni Islamist insurgents.
Security is especially important ahead of a March 7 national election, with Shi'ite Prime Minister Nuri al-Maliki struggling to defend his claim to having quelled violence in Iraq after a spate of devastating bombings in the capital in recent months.
Some 20,000 members of Iraq's security forces formed cordons around Karbala, vehicles were banned and 1,000 snipers were perched on the roofs of buildings. Troops stood watch with bomb-sniffing dogs and the wands used to detect explosives.
"It was difficult to get in, but it's better than having bombings and lots of victims," said Muhammed Abu Sajad, a pilgrim at Karbala.
Pilgrims, most dressed in black, thronged the streets leading to Karbala's golden-domed shrines of Imam Hussein and Imam Abbas, many beating their chests in mourning and chanting accounts of Hussein's death on the battlefield at Karbala.
For Shi'ites, Hussein's death symbolizes confronting tyranny in the face of overwhelming odds. It is also a reminder of the rift with Sunnis, who do not revere Hussein as Shi'ites do, over the Prophet Muhammad's succession.
Years of sectarian warfare after the U.S. invasion almost tore Iraq apart.
Despite the tight security there have been sporadic, mostly small-scale attacks on pilgrims in recent days. One roadside bomb killed four pilgrims and wounded 18 in Tuz Khurmato, north of Baghdad, early today. Another killed two pilgrims and wounded eight in Baghdad on December 26.
The vast number of pilgrims streaming into Karbala on foot from across Iraq makes it very difficult to ensure security during Ashura.
Shi'ite religious gatherings have been occasions of major bloodshed in the past. In 2005, rumors of a suicide bomb attack during a Shi'ite festival panicked pilgrims on a bridge, and the resulting stampede killed about 1,000 people and clogged the river below with corpses.