Tens of thousands of Shiites gathered in the holy city of Karbala. According to police, a cache of explosives were uncovered and four insurgents arrested for allegedly planning an attack on the pilgrims.
In December, more than 50 pilgrims died in a series of bombings in the city, 80 kilometers south of Baghdad, and in March 2004 at least 181 people died in coordinated bombings of Shiite pilgrims in Karbala and Baghdad. Both attacks were blamed on Sunni extremists.
Karbala also was the scene of heavy fighting in April 2004 between the Shiite militia of anti-U.S. cleric Muqtada al-Sadr and multinational forces.
The explosives cache found near Karbala was discovered late Sunday on farm land on the outskirts of the city, said police spokesman Rahman Mishawi.
Three "non-Iraqi Arabs" were arrested, Mishawi said, along with a man armed with several hand grenades who was caught walking with a procession of pilgrims.
Massive security precautions have been taken in Karbala and concern has deepened after Sunni terrorist leader Abu Musab al-Zarqawi on Wednesday declared "all-out war" against the Shiite majority.
Officials said they expected more than a million people to gather Monday for celebrations marking the birth in 868 A.D. of the 12th imam, Mohammed al-Mahdi, who vanished without explanation. Shiites believe he will return.
Britain, which has about 9,000 forces in southern Iraq, will keep its troops in the country as long as they are required and could send more, British Defense Secretary John Reid said on Sunday.
In the meanwhile since Wednesday, when 14 suicide bombs exploded in Baghdad in the bloodiest day in the capital since the war began, a staggering wave of insurgent violence has killed at least 250 people and wounded hundreds more nationwide.
Shiites have suffered the brunt of the attacks, which al-Zarqawi said were in retaliation for the Iraqi-U.S. military operation against the insurgent stronghold of Tal Afar, a northern city near the Syrian border, the AP reports.