More than 130 people were killed in and around the Iraqi capital, Baghdad, on the country's bloodiest day this year.
The attacks highlight the challenge faced by US troops
Eighty-eight people died and 160 were injured in a double car bombing at a second-hand clothes market in Baghdad.
A further 12 died in a bomb and mortar attack in the nearby city of Baquba, while Iraqi police found 29 bodies in and around the capital.
The attacks came as the first of over 21,000 extra US troops arrived in Baghdad on a mission to boost security.
The 3,200 troops are the advance guard of an increase ordered by President George W Bush earlier this month.
Choked with traffic
The first big attack on Monday came in the Haraj market, which sells second-hand clothing and DVDs, shortly after midday (0900 GMT). Columns of thick smoke immediately covered the area.
One unconfirmed account of the attacks said that a bomb in a parked car was followed seconds later by a suicide bomber ploughing his car into the terrified crowd.
At least 12 vehicles were set ablaze, said a photographer for the AFP news agency at the scene.
He said there were so many victims that the wounded were piled up alongside the dead on wooden market carts.
Bodies could be seen covered in blue sheeting outside a Baghdad mortuary, while doctors at al-Kindi Hospital worked frantically to save the lives of the badly injured.
Relatives of the dead could be seen crying and weeping nearby.
The BBC's Mike Wooldridge in Baghdad says the market is popular with the many Baghdad residents on low incomes.
It is also a busy transport junction, and was choked with traffic at the time, he added.
At about 1700 (1400 GMT) there was a second attack, this time on a market near the town of Baquba, north-east of Baghdad.
Police said a bomb went off, followed by a mortar attack, leaving at least 12 civilians dead and 26 injured.
Lieutenant Ahmed Mohammed told AFP the bomb was hidden in a vegetable cart and exploded as people shopped late in the day at Khalis market.
Five minutes later further carnage was wrought in the shape of an incoming missile.
Elsewhere, a teacher was killed in west Baghdad and at least one woman died in a mortar attack in the south of the city.
Late in the day, police confirmed that they had found 29 unidentified bodies with gunshot wounds.
The attacks are seen as highlighting the challenges faced by US forces as they prepare to try to rein in the Sunni and Shia fighters who have been carrying out deadly tit-for-tat attacks.
Previous attempts to stop the killings in the capital have failed, in part, analysts say, because coalition and Iraqi troops have not stayed in an area once insurgents have been cleared.
Under the new plans, once an area is taken, the extra US troops will stay behind, backing up Iraqi forces to hold the area.
Doubts, however, remain as to whether there will be enough extra troops to stabilise a city of more than six million people, while among Baghdad residents there are fears the presence of the troops will simply inspire more violence.
US troops have suffered significant losses in recent days. On Saturday, 25 soldiers were killed - one of the worst days for the US army since the invasion.