Story highlights The deadliest of the 10 strikes occurred in Hilla

Most of the victims were among thousands of Shiite pilgrims traveling to Kadhimiya

The pilgrims will commemorate the death of Imam Moussa al-Kadhim

The shrine to al-Kadhim in Baghdad is one of Shiite Islam's holiest sites

A string of bombings killed dozens across Iraq on Wednesday, a show of sectarian tension in the country's deadliest day since January.

Shiite pilgrims trekking and driving to a shrine in Baghdad accounted for most of the 61 people killed in the series of roadside and car bombs in northern and central Iraq, police officials said. The attacks left 153 others wounded.

"I am deeply shocked and utterly dismayed by the despicable attacks across Iraq today that have claimed the lives of scores of Iraqis, including many pilgrims, and have injured dozens more," said Martin Kobler, the U.N. secretary-general's special representative for Iraq.

"The scale of the violence is disturbing. I urgently appeal to the government to address the root causes of the violence and terrorism that are causing so much suffering and pain to the Iraqi people."

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En route from across the country, including the Shiite heartland of southern Iraq, pilgrims have been headed to the Imam Kadhim shrine in the Kadhimiya neighborhood of Baghdad. The event culminates Saturday when the faithful commemorate the death of Imam Moussa al-Kadhim, one of 12 revered imams in Shiite Islam.

Strikes occurred in 10 locations, with the deadliest in Hilla, south of Baghdad, where at least 20 people died in car bombs.

Bombings occurred in Kadhimiya and other Baghdad neighborhoods. Strikes also occurred in Balad and Kirkuk, north of the capital, and in Hindiya and Madaan, south of Baghdad.

The violence sparks fears of a renewal of the fighting between Sunnis and Shiites in Iraq in the last decade, a longtime animosity intensified by the U.S. invasion of Iraq and the toppling of the Saddam Hussein regime.

Most people in Iraq are Shiites, but under Hussein, Sunnis held a great deal of power despite their minority status.

Shiites gained the upper hand politically after Hussein was overthrown by a U.S.-led invasion in 2003. Sunnis felt disenfranchised amid that political backdrop, and many backed insurgent actions against the government.

As a result, Sunni-Shiite violence exploded in the early 2000s. While full-blown sectarian violence had ebbed by the end of the Iraq War, Sunni extremists continued to launch attacks against Shiite pilgrims over the years.

In January, for example, attackers killed at least 60 people when they targeted Shiites on a pilgrimage to Karbala leading up to Arbaeen, another holy Shiite period. That day marked the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Mohammed, who was killed in a seventh century battle in the Iraqi city.

Wednesday's violence also heightened political tensions as several major political groups, including Shiite, Sunni and Kurdish factions, pursue a no-confidence vote in Iraq's Council of Representatives against Prime Minister Nuri al-Maliki.

One of the attacks, in Kirkuk, targeted the offices of a Kurdish party.

Al-Maliki, a Shiite politician with a predominantly Shiite following, held an urgent meeting with high-level security officials after the attacks. He warned that "political differences that stand at a certain point may reflect negatively on the security situation."

His political opponents argue that he is unable and unwilling to cobble out a power-sharing deal.

Kobler, the U.N. official, traveled to the Kurdish region Tuesday to discuss the political situation in Iraq with President Jalal Talabani and Masoud Barzani, president of the Kurdistan region. Kobler urged "all parties in Iraq to engage in inclusive dialogue and to resolve their differences."

Other violence has rocked the country this year.

A spate of car bombings on the ninth anniversary of the U.S.-led invasion in March, and days before the nation hosted a meeting of Arab leaders, left nearly 50 people dead and scores wounded, officials said. And on one February day, a wave of attacks left nearly 50 people dead and dozens wounded.