Several people have been killed in Iraq in bomb attacks targeting police and pilgrims, according to police and medical sources.
No group immediately claimed responsibility for Wednesday's attacks, which coincided with the holy ritual of Ashura, when Shia Muslims commemorate Imam Hussein who died more than 1,000 years ago.
The 10-day ritual ends on Thursday. The AFP news agency reported 23 deaths while Reuters put the figure at 19 citing its own sources.
Security personnel are also a prime target for Sunni Islamist fighters linked with al-Qaeda, which seeks to destabilise
Iraq's Shia-led government and foment inter-communal conflict.
In the deadliest attack, 10 people were killed when a suicide bomber drove a lorry packed with explosives into a police checkpoint in al-Alam, a town near Tikrit, police said.
Anti-government fighters, many of them linked to al-Qaeda's Iraqi affiliate, have gained a foothold in Tikrit.
Near the city of Baquba, three roadside bombs exploded near a group of Shia pilgrims commemorating Ashura, killing nine, medics and police sources said.
Violence began to ease after al-Qaeda in Iraq was forced underground in 2007, but it is rising again, with more than 7,500 civilians killed so far this year, according to monitoring group Iraq Body Count.
The civil war in neighbouring Syria has drawn Sunni Islamists from across the region and beyond into battle against President Bashar al-Assad, who is an ally of Shia-led Iran.
Al-Qaeda in Iraq earlier this year joined forces with its Syrian counterpart to form the Islamic State in Iraq and the Levant, which has claimed responsibility for attacks on both sides of the border.
In past years, pilgrims have been targeted by bombings, including serial attacks the day before Ashura in 2011 that killed 28 people.
As a result, security measures are stepped up, with more than 35,000 soldiers and policemen currently deployed to Karbala and surrounding areas, with concentric security perimeters barring vehicles from entering the city while helicopters hover overhead.