Two dozen children were among the 155 people killed after two truck bombs exploded in one of Baghdad's safest areas on Sunday.

An official at the hospital where the children's bodies were brought says they were on a bus leaving a daycare center next to the Justice Ministry when the attack happened. The bus driver was also killed, and six kids injured.

VIDEO: Baghdad Blasts

A local police official confirmed the deaths of the children in Sunday's attack. Both officials spoke on condition of anonymity Monday because they were not authorized to talk to the media.

Iraqi security forces blocked streets around the capital Monday and carried out intense searches at checkpoints a day after the country's worst attacks in more than two years. The attacks targeted the Justice Ministry and Baghdad Provincial Administration in the heart of the capital, calling into question Iraq's ability to protect itself as it prepares for January elections and the U.S. military withdrawal.

"Today, we came to work despite the fear inside us," said Siham Abdul-Karim, 49, an employee of the Culture Ministry located near the site of the bombings and surrounded by police checkpoints on Monday. "We all wonder how could car bombs could reach these institutions."

The death toll rose to 155 on Monday as Baghdad residents buried the dead. About 500 people were injured, authorities said.

SLIDESHOW: Baghdad Bombings (Warning: Graphic)

The initial investigation suggested the vehicles, each packed with thousands of pounds of explosives, might have passed through some security checkpoints before hitting their destination, said Maj. Gen. Qassim al-Moussawi, a spokesman for the city's operations command center. Authorities have said they are also checking security cameras in the area.

Interior Minister Jawad al-Bolani said 76 people have been arrested so far, but he did not provide information on who they were or how they are believed to be connected to the horrific crimes, which took place just hundreds of yards from the heavily fortified Green Zone.

"This is a terrorist act," al-Bolani said. He called on all the political forces to cooperate and assist the Iraqi security forces.

The street where the blasts occurred had just been reopened to vehicle traffic six months ago. Shortly after, blast walls were repositioned to allow traffic closer to the government buildings — all measures hailed by Prime Minister Nouri al-Maliki as a sign that safety was returning to the city.

Al-Maliki has staked his political reputation and re-election bid on his ability to bring peace to the country but as grief turned into anger on Monday people questioned the government's recent security policies — ominous signs for al-Maliki's re-election bid.

"This explosion made people furious," said Ahmed Hassan, an employee at the Ministry of Education. "People will not re-elect this government."

The pickup truck that exploded near the Justice Ministry was carrying 2,205 pounds (1,000 kilograms) of explosives, the initial investigation found. The second pickup truck that went off near the Baghdad Provincial Administration building, was carrying 1,543 pounds (700 kilograms) of explosives.

The explosives were attached to the vehicles and hidden below the seats, al-Moussawi said.

Iraqi health and security officials confirmed the death toll. They spoke on condition of anonymity because they were not authorized to release information to the media.

The coordinated bombings were the deadliest since a series of massive truck bombs in northern Iraq killed nearly 500 villagers from the minority Yazidi sect in August 2007. In Baghdad itself, it was the worst attack since a series of suicide bombings against Shiite neighborhoods in April 2007 killed 183.

Massive car bombs have been the hallmark of the Sunni insurgents seeking to overthrow the country's Shiite-dominated government. Iraq has accused members of the outlawed Baath Party living in neighboring Syria of being behind another series of deadly bombings in August that also targeted government buildings and killed more than 100 people.

The Associated Press contributed to this report.