BAGHDAD — U.S. and Iraqi authorities freed 500 prisoners Thursday from a detention system strained to the limit by thousands of new suspects taken in campaigns to secure Baghdad and surrounding areas.

The prisoner release — in an event organized by the U.S. military — was intended to highlight the progress in regaining control of former extremist strongholds since the arrival of 30,000 additional U.S. troops earlier this year. But it also did little to relieve pressures on the compounds holding 25,800 Iraqis.

U.S. officials worry the overcrowded detention camps are sapping resources and will overwhelm Iraq's struggling justice system. Sunni leaders also complain that their community has been heavily targeted by Iraq's Shiite-dominated security forces — claiming that nearly 90 percent of the detainees are Sunni.

The head of Iraq's Shiite-led government, Prime Minister Nouri al-Maliki, told the freed prisoners they serve as examples of reconciliation as the nation tries to heal deep rifts between the majority Shiites and Sunnis, who lost their privileged status with the fall of Saddam Hussein.

"Dear brothers, let's cooperate to shut down these prisons and write a new page of laws with the power of justice," al-Maliki told the detainees, who wore civilian clothes and sat in white plastic chairs under the Baghdad sun.

The backgrounds of the prisoners, including any suspected militant links, were not announced during the event at Camp Victory — a giant American military base that stretches across western Baghdad and includes the country's main airport and several of Saddam Hussein's former palaces, where the U.S. command now has offices.

To the west of Baghdad, seven more decomposed bodies have been unearthed in Iraq's once restive Anbar province, police said.

The victims, who were blindfolded and handcuffed, were found Wednesday in the Lake Tharthar area during a joint operation with the U.S. forces, a police officer said on condition of anonymity because he feared reprisal.

Last Saturday, Iraqi soldiers found 22 bodies in the Lake Tharthar area, about 60 miles northwest of Baghdad.

On Wednesday another mass grave was found amid brush near a school in Hashimiyat, west of Baqouba, 35 miles northeast of Baghdad. The city is the capital of Diyala province, where al-Qaida in Iraq is believed to have a strong presence.

Many of those bodies also were handcuffed and blindfolded, police said. They likely were passengers kidnapped at fake checkpoints on a nearby road leading to Baqouba, a dangerous route dubbed the "road of death."

In southern Iraq, a senior politician in Iraq's second-largest city survived an assassination attempt Thursday when a roadside bomb exploded next to his convoy, police said.

Qahtan al-Mousawi, general director of the Basra education department, was traveling in a two-car convoy through central Basra when the bomb went off around 8 a.m., a police officer said on condition of anonymity because he was not authorized to release the information.

Two of al-Mousawi's bodyguards and two pedestrians were wounded, he said.

Al-Mousawi is an influential member of the Supreme Islamic Iraq Council, Iraq's largest Shiite party. The attempt on his life is the latest episode in escalating tensions between Shiite rivalries in the city, about 340 miles southeast of Baghdad.

On Wednesday, the city's police chief, Maj. Gen. Jalil Khalaf, also survived a bomb attack on his convoy. It was the second such attack on Khalaf in less than a week.

Meanwhile southeast of Baghdad, a policeman was killed and two others injured when a bomb struck their convoy early Thursday, police said. The attack occurred at 8:30 a.m. in the Jisr Diyala area, about 10 miles southeast of the capital, they said.