Maya Alleruzzo, Associated Press
U.S. Army soldiers react after coming under fire in western Mosul. One soldier, at right, was wounded.

BAGHDAD — U.S. authorities freed 500 Iraqi prisoners Thursday in an ongoing push to empty American jails of detainees no longer deemed a threat. But the military says it's still holding 25,800 Iraqis waiting to face charges or be given freedom.

The latest release provided only small relief to a detention system strained to the limit by about 17,000 new suspects captured this year in campaigns to secure Baghdad and its surrounding belts, the military said. U.S. officials worry the overcrowded detention camps are sapping resources and will overwhelm Iraq's struggling justice system.

The periodic releases are seen as both a symbolic gesture to highlight increased security and a needed safety valve. About 6,300 detainees have been released since January.

The ceremony — held behind concrete blast walls at Camp Victory, a sprawling U.S. base that contains several of Saddam Hussein's former palaces — coincided with other signs of progress in regaining control of former extremist strongholds since the arrival of 30,000 additional U.S. troops earlier this year.

In Washington Thursday, House Democrats, under pressure to support the troops but end the war, said Thursday they would send President Bush $50 billion for combat operations on the condition that he begin withdrawing troops from Iraq.

The proposal, similar to one Bush vetoed earlier this year, would identify a goal of ending combat entirely by December 2008. It would require that troops spend as much time at home as they do in combat, as well as effectively ban harsh interrogation techniques like waterboarding.

Both U.S. troops deaths and civilian casualties have dropped in recent months. U.S. forces, meanwhile, have made important alliances with Sunni clan leaders to battle extremists such as al-Qaida in Iraq.

But Sunni leaders also complain members of their sect make up the vast majority in both U.S. and Iraqi custody.

Flanked by U.S. soldiers, Prime Minister Nouri al-Maliki addressed the detainees, many wearing identical plaid shirts. "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 men, who sat in rows of white plastic chairs under the Baghdad sun.

The backgrounds of the prisoners, including any suspected militant links, were not announced. The military issued a press release saying only that the detainees are "no longer an imperative threat to Iraqi/coalition forces and the security of Iraq."

One of the men, who identified himself as Jumaa Khashan, a Sunni from Ramadi, said he was arrested in 2005 on his way to visit relatives in the neighboring town of Khalidiya.

"At first, the treatment was bad ... but this year the treatment became better," Khashan said. "I hope that Iraqis will renounce violence and work together to build a new Iraq."

Another Sunni detainee also from Anbar province, Adel Khalaf, said he plans to return to Fallujah to look for his family.

"I will go back to see what happened to them. I haven't heard anything from them for the past 11 months," Khalaf said. "The second thing I'll do is try to live a normal life."

Meanwhile, there were signs of more unrest in Iraq's second-largest city, Basra, where Shiite factions and Iraqi security forces have been battling for power after the withdrawal of British forces to the airport on the edge of the city.

The general director of the Basra education department, Qahtan al-Mousawi, survived an attempt on his life when a roadside bomb exploded next to his convoy in central Basra around 8 a.m., police said. It was the third assassination attempt on top officials in the city in less than a week.

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

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

The city's police chief, Maj. Gen. Jalil Khalaf, survived bomb attacks on his convoy twice in the past five days.

In western Iraq, seven more decomposed bodies were unearthed Thursday in the Lake Tharthar area of Iraq's once restive Anbar province, where a mass grave was discovered five days earlier.

Iraqi soldiers found the latest victims — blindfolded and handcuffed — during a joint patrol Wednesday with U.S. forces, police said.

Last Saturday, Iraqi soldiers found 22 bodies in the same region, about 60 miles northwest of Baghdad.

On Wednesday, another mass grave was found amid brush near a school in Hashimiyat, west of Baqouba, about 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."

Scattered violence continued. At least 19 people were killed or found dead across Iraq, including in the Anbar mass grave.

They included a policeman killed by a roadside bomb southeast of Baghdad, and a civilian woman killed when a suicide bomber struck a Kurdish political party's office in a predominantly Christian area near Mosul, about 225 miles northwest of Baghdad.