BAGHDAD — At least 17 people were killed in wave of bombings across Iraq Monday, as authorities tightened security measures ahead of a major Shiite religious ceremony.

The deadliest attacks occurred in the southern city of Hilla, 60 miles south of the capital of Baghdad.

Seven people were killed and four wounded when gunmen blew up three houses, security sources told the German news agency dpa. The attack followed a car bombing that killed one person and wounded 17 near a Shiite mosque in the city.

In northern Iraq, five police officers were killed and seven injured in two attacks in Kirkuk city, while one person was killed in the northern city of Mosul when a roadside bomb targeting a police patrol went off.

Three people were killed and 16 injured when a car bomb exploded in the Baghdad neighborhood of Karadah, the Sumaria News website reported.

The attacks came as Iraqi police as police and army forces tightened security measures to protect thousands of pilgrims, who began arriving in the country for the Arbaeen pilgrimage, which peaks on Thursday.

Each year millions of Shiites make the pilgrimage to the southern city of Karbala to commemorate 40 days after the slaying of Imam Hussein bin Ali, the grandson of the prophet Mohammed.

The Shiite-dominated province of Najaf, in central Iraq, announced Wednesday and Thursday a public holiday to celebrate the Arbaeen ritual.

Pilgrims usually visit several holy shrines in the country before arriving in Karbala.

The annual festival has been the target of deadly attacks blamed on al-Qaida and radical insurgents in recent years.

Deadly attacks have been on the rise in Iraq since U.S. troops left the country in December, raising fears of a return to the sectarian tensions that drove Iraq close to civil war in 2006 and 2007.

The government, meanwhile, has been stuck in political deadlock, with thousands of Sunni Muslims staging a series of protests against the Shiite-dominated Cabinet and to condemn what they said were attempts by Prime Minister Nouri al-Maliki to marginalize Sunni politicians.

Al-Maliki’s relations with Iraq’s Sunnis soured last year when authorities issued an arrest warrant against the Sunni Vice President Tariq al-Hashemi on terrorism charges.

Al-Hashemi denied the accusation and fled to Turkey. An Iraqi court in September sentenced him in absentia to death.