The attack, which happened at 9.30am (0630 GMT) on Monday, is the deadliest single attack in Iraq in a year.
“We spoke to our branch office there an hour ago and they told us there are more than 100 killed,” Firdus al-Abadi, a Red Crescent spokeswoman in Baghdad, said.
“We are sending an emergency load of medicine and doctors to al-Hilla right now.”
The Red Crescent’s office in al-Hilla is not far from the site of the explosion, she said.
The victims were job seekers queuing up to get medical identification cards needed to fill out applications for state jobs.
The bomber managed to penetrate the barricades and blow up his car in front of the building, Iraqi journalist Talib al-Janabi told Aljazeera.
US and Iraqi forces closed the roads while ambulances and firefighters rushed to the scene to evacuate casualties and put out fires, he said, adding that the attack coincided with another car bomb in al-Musayib city, 60km north of al-Hilla.
During the past week, at least four car bombs and five explosive devices have gone off in the area north of al-Hilla, al-Janabi said.
Meanwhile, armed men killed four people and wounded two in the northern Iraqi city of Mosul, the US military said late on Sunday.
Sunday’s drive-by shooting followed a bombing near Mosul in Hamam al-Alil that killed eight people.
Mosul has become a centre of efforts to defeat Iraq’s anti-government fighters after US marines took over the fighters’ former stronghold of Falluja in November.