There have been four separate car bomb attacks in Iraq, two near the northern city of Mosul and another two near the capital, Baghdad.
Reports say 41 people have been killed in total and at least 200 wounded on Monday.
The explosions took place in mainly Shia areas. Last week, a string of bombings targeting Shi'ites across Iraq killed 44 people.
US troops withdrew from towns and cities in June. They are due to leave Iraq by 2012.
Prime Minister Nouri al Maliki has warned of an upsurge of violence ahead of the next elections in January.