Thursday's blast near the Imam Hussein shrine in central Karbala, 80km south of Baghdad, killed 49 people and injured 52, police said.
Hospital sources said a bomber detonated his explosive belt on a crowded street near one of the city's main Shia shrines on Thursday.
In the city of Ramadi, a second bomber blew himself up on Thursday outside a police recruiting centre, killing at least 60 people and injuring another 70, police and hospital officials told Reuters.
The attack took place at a police screening centre in Ramadi, 115km west of Baghdad.
Earlier, Mohammed al-Ani, a doctor at Ramadi General Hospital, said at least 30 people had died.
The US military did not have a casualty figure, but said that a suicide bomb exploded at 0755 GMT near the Ramadi Glass and Ceramics Works "where screening for Iraqi police officers was taking place".
Also on Thursday, two roadside bombs killed seven US soldiers, the US military said.
Five were killed by a roadside bomb in Baghdad and two were killed by a similar device near the southern city of Najaf.
Furthermore, fighters sabotaged an oil pipeline near the northern city of Kirkuk, causing a huge fire.
Ali al-Hasnawi, an Iraqi journalist, told Aljazeera from Karbala that the explosion was one of the most devastating in the city in two years.
Al-Hasnawi quoted the director of al-Hussein hospital as saying that more than 50 people were killed and 85 injured.
Ambulances and civilian cars are still transferring casualties to Karbala hospital, he said, adding that security forces sealed off the area and imposed a security cordon to prevent media coverage.
The attack in Ramadi took place
at a police screening centre
It was the second bomb attack in Karbala in as many days after over a year of relative calm.
Iranians were among the casualties, he said. Iranian pilgrims have recently started to return to Shia holy sites in Iraq.
Pools of blood could be seen at the blast site, and television images showed men ferrying the wounded in push carts.
The bomber appeared to have set off the explosion only about 30 metres from the Imam Hussein shrine in a busy pedestrian area surrounded by shops.
Karbala has been relatively free of violence since December 2004, when seven people were killed and 31 wounded in an attack.
Thursday's attacks follow a heavy day of violence on
Wednesday, when at least 53 people were killed around Iraq,
including 32 killed by a bomber at a Shia funeral
east of Baquba.