Updated

Bombers struck Shiite pilgrims Saturday for a third consecutive day, killing six people in the latest in a series of attacks apparently aimed at stoking sectarian tension.

The attacks have targeted pilgrims headed for the Shiite city of Karbala, where hundreds of thousands of people have gathered for festivities that culminate Sunday morning.

No group has claimed responsibility, but assaults on Shiite civilians have been carried out for years by Sunni extremists such as Al Qaeda in Iraq.

The latest attack occurred about 9 a.m. Saturday when a car bomb exploded in the north Baghdad neighborhood of Shaab as pilgrims were boarding minibuses bound for Karbala, 50 miles to the south.

On Friday, a passenger van packed with explosives blew up at a bus station in Balad, north of Baghdad. The Balad hospital director, Qassim Hatam al-Qaisi, said nine people were killed and 40 were wounded.

A female homicide bomber killed 18 pilgrims Thursday when she detonated explosives resting by the side of a road in Latifiyah, 20 miles south of the capital.

The attacks have heightened concern that extremists are seeking to re-ignite the firestorm of sectarian massacres that pushed Iraq to the brink of civil war two years ago before thousands of American reinforcements were rushed to the country.

The violence, however, did little to deter Shiite pilgrims who descended on Karbala for the festival, known as Shabaniyah, which marks the birth of Imam Mohammed al-Mahdi — known as the "Hidden Iman" — a Shiite saint who disappeared in the ninth century.

Devout Shiites believe he will return some day to usher in peace and harmony in the world.

"I am determined to celebrate this occasion despite all the hardships such as heat and security concerns," said Ahmed Hussein, 21, a day laborer who walked for three days from Baghdad to Karbala.

"I feel that the situation is better now because I notice that there is more security. I am happy to be here today," he said.

Pilgrims moved Saturday through the center of Karbala near the two main, golden domed shrines. Helicopters hovered over the area, and Iraqi snipers stood guard on the roofs watching for any sign of trouble.

Some worshippers placed candles on small pieces of cork and floated them on a small canal that runs through the city.

Women in flowing black robes were searched by female guards at separate checkpoints leading into the center of the city. Insurgents are increasingly using women to stage attacks because they can more easily hide explosives under their abayas and men are prevented by custom from searching them.

Guards confiscated posters of Shiite religious leaders for fear they might provoke attacks by followers of rival clerics, and mobile phones were banned because they could be used to trigger bombs.

Mohammed Muzhir, 39, a government employee from the southern city of Nasiriyah, said extensive security checks along the route made it difficult to reach Karbala.

"I hope that these measures will prevent the terrorists from carrying attacks and bombings in the holy city," he said.

In Baghdad, Iraqi soldiers Saturday arrested the head of a U.S.-funded Sunni group who was accused of "supporting terrorism" in a series of raids in the western Baghdad district of Jihad.

An Iraqi officer, who declined to be identified because he was not authorized to speak to the media, identified the suspect as Tahir Abdullah al-Hamdani, the head of the so-called awakening council in Jihad.

The U.S. military confirmed that a leader of the group was arrested but declined to identify him or give more details.

American commanders have said the Sunni revolt against Al Qaeda in Iraq was a key factor in a sharp decline in violence over the past year, but the movement has been troubled by friendly fire incidents and concerns about infiltration.

AP Television News video showed Iraqi soldiers combing the largely empty residential area and a blindfolded man sitting cross-legged next to three rows of Kalashnikov assault rifles.

Iraqi army officer Col. Ali Abboud Thamir said the raid was aimed at clearing the area of extremists so about 240 displaced families could return next week.