BAGHDAD — A man with explosives strapped to his body blew himself up in a crowd, bombers struck a southern city and gunmen sprayed fire on security checkpoints in attacks Monday that killed at least 100 people — most of them in Shiite areas — in Iraq’s deadliest day this year.

Officials were quick to blame insurgents linked to al-Qaida in Iraq for the shootings in the capital, saying the militants were redoubling efforts to destabilize the country at a time of political uncertainty over who will control the next government.

The relentless cascade of bombings and shootings hit at least 10 cities and towns.

Most of the day’s casualties were in two Shiite-dominated cities where wounded victims screamed their fury at the government for failing to protect them.

The worst violence hit the Shiite city of Hillah. Two parked car bombs near a textile factory exploded as workers were leaving the factory around midday, a police spokesman said.

Then as rescuers and workers were trying to help the injured, a suicide attacker with explosives strapped to his body blew himself up in the crowd.

At least 45 people were killed and dozens more injured, according to a hospital director.

In Basra, another Shiite city, three bombs, including one that targeted a marketplace, killed at least 16 people, officials said.

More than two months after the March 7 election, Iraq’s main political factions are still struggling to put together a ruling coalition. Prime Minister Nouri al-Maliki’s Shiite bloc has tried to squeeze out election front-runner Ayad Allawi — a secular Shiite heavily backed by Sunnis — by forging an alliance last week with another religious Shiite coalition. The union, which is just four seats short of a majority in parliament, will likely lead to four more years of a government dominated by Shiites, much like the current one.

Sunni anger at Shiite domination of successive governments was a key reason behind the insurgency that sparked sectarian warfare in 2006 and 2007.

If Allawi is perceived as not getting his fair share of power, that could in turn outrage the Sunnis who supported him and risk a resurgence of sectarian violence.