A suicide car bomber killed 17 Shiites at a teeming Sadr City market on Wednesday, while gunmen in a predominantly Sunni neighbourhood of Baghdad shot up a convoy of democracy workers in an ambush that took the lives of an American woman and three bodyguards.
The attack on the marketplace came one day after car bombings killed scores of university students just two miles away, indicating that Al-Qaeda-linked fighters are bent on a surge of bloodshed.
There had been a relative lull in Baghdad violence since the first of the year. Prime Minister Nouri al-Maliki said on Wednesday about 400 militiamen loyal to radical Shiite cleric Moqtada al-Sadr had been arrested over the past several weeks.
It was the first time al-Maliki detailed specific arrests of members of the Mahdi Army, which has been blamed for much of the sectarian killing in the past months.
Yassin Majid, a senior al-Maliki adviser, said reports that dozens that senior militia leaders had been detained were incorrect.
A Baghdad Mahdi Army commander, meanwhile, said US and Iraqi troops launched a major campaign on Tuesday in Um al-Maalef, a Shiite neighborhood in south Baghdad.
"They detained every man who was able to carry weapons. We heard from our people in the area that about 400 people were detained," said the militia commander on condition of anonymity because senior figures in the group are not permitted to give their names.
He said that in December US troops had killed one of the Mahdi Army's top commanders, known as Abu al-Sudour, in Sadr City.
State television also reported that at least 100 insurgents were killed on Wednesday in clashes with Iraqi troops in a predominantly Sunni region northeast of Baghdad.
Troops captured dozens of insurgents and seized large amounts of ammunition, the state-run Iraqiya channel said, quoting police.
The fighting reportedly took place near the district of Balad Ruz, 45 miles northeast of the capital. The report couldn't immediately be confirmed with Iraqi authorities.
An Iraqi army officer, who spoke on condition of anonymity because of security concerns, said the attack on the Western convoy took place in Yarmouk, a predominantly Sunni neighborhood in western Baghdad.
The three-car convoy belonged to the Washington-based National Democratic Institute, according to Les Campbell, the not-for-profit group's Middle East director.
He said the four dead included an American woman along with three security contractors -- a Hungarian, a Croatian and an Iraqi.
Two others were wounded, one seriously, Campbell said by telephone from Washington. Their names were withheld until their families could be notified.
"It appeared to be an attack with fairly heavy weapons, we don't know what kind," Campbell said.
"We have some information that a firefight ensued. Our security company responded to the attack." Campbell said the ambush took place at midday as the group returned from a program elsewhere in Baghdad.
Few foreigners and even fewer women have been caught up in Iraq's recent wave of violence as many Western groups have left and those who remain have tightened security and curtailed their movements after a series of kidnappings and beheadings.
The last known American female civilian to be killed was Marla Ruzicka, a 28-year-old rights activist from California who died in a car bombing in April 2005.
Al-Maliki did not give a start date when he announced plans for a new drive to tame the violent capital -- the third attempt since he took office May 20.
But US and Iraqi reinforcements have started to arrive in Baghdad, and it was expected to begin in about two weeks.
The marketplace explosion took place just before 4 p.m. near a popular commercial area in Sadr City, a sprawling Shiite district of some 2.5 million people in eastern Baghdad.
The blast shattered the windows of nearby shops and restaurants, and blood pooled in the street.
Angry Iraqis surrounded the charred mass of twisted metal, all that was left of the explosives-packed car.
They tipped the remains on its side and picked off pieces of blackened upholstery.
At least 17 people were killed and 33 people were wounded, police said.
In many parts of the capital, streets were crowded with cars and minivans carrying wooden caskets of the victims from Tuesday's car bombings, which killed at least 70 people and wounded more than 130 at Al-Mustansiriya University.
Many vehicles were headed to the holy city of Najaf where Shiites prefer to bury their dead. Other victims were taken to a Sunni cemetery in central Baghdad. The students were from all the country's religious sects.
Hussein Mohammed, a lecturer in the university's French language department, said classes were canceled for two days while workers cleared the debris.
"We are trying to heal our wounds and start again," he said.
The Iraqi parliament stood for a moment of silence and lawmakers and students demanded stepped-up security for schools and universities.
Al-Maliki announced the new security drive on January 6, four days before President Bush detailed his version of the plan with an announcement that he was sending 21,500 more US troops to Iraq.
There have been concerns that insurgents would just slip out of the capital to wait out the offensive. Some appear to have left, given the spike in violence in northern Iraq, where Sunni militants have retreated in the past.