A car bomb exploded in a largely Shiite neighborhood Wednesday, killing at least 16 people, just as Defense Secretary Robert Gates visited the capital and said a secure and stable Iraq was within reach.

It was the deadliest of four bombs in Iraq on Wednesday that killed a total of 25 people. Earlier, a blast went off in the northern city of Mosul, where Gates had landed on his sixth trip to Iraq.

Gunfire and sirens followed the bombing in Baghdad's Karradah neighborhood, and a plume of smoke rose in the sky.

Karradah has frequently been hit by car bombings, but attacks have tapered off in recent months. Cafes, shops and kebab stands have flourished anew in the neighborhood.

Firas Adel, who owns a clothing store about 400 yards from the site of the explosion, said terrified bystanders fled when the bomb went off.

"The police prevented us from getting to the site, but I saw the wounded screaming for help, including a teenage friend of mine who lost both his feet," Adel said. "There are people who want to end the security improvements and bring back chaos."

Police and hospital officials said at least 16 people were killed and 32 wounded in the explosion, which took place across the Tigris River from the Green Zone shortly before Gates' news conference with Iraqi Defense Minister Abdul-Qader al-Obeidi.

"I believe that a secure, stable Iraq is within reach," Gates said, adding however: "We need to be patient."

He said that Iraqis who have been fighting al-Qaida and other military forces on the local level must be integrated into the formal Iraqi security forces.

Gates noted a decline in overall violence in Iraq during recent months, that has led to "a growing sense of normalcy and hope." But he acknowledged increasing militant and terrorist activities in northern Iraq.

Army Col. Tony Thomas, a brigade commander, said senior commanders in the north are looking for additional U.S. troops and also would like the return of 1,400 Iraqi troops sent to Baghdad to provide "more combat power" to help stabilize areas such as Diyala province, Mosul and Samara.

The U.S. is pressing Iraqi leaders to take advantage of the improved security to make the political reforms needed to stabilize the fledgling democracy.

The American military also announced the deaths of three soldiers from a "complex attack" on Tuesday involving a roadside bomb and small arms fire north of Baghdad, the U.S. military said. A fourth soldier was wounded, it said.

At least 3,886 members of the U.S. military have died since the war began in 2003, according to an Associated Press count. The figure includes eight military civilians.

Baghdad's Karradah neighborhood has frequently been hit by car bombings, although attacks have tapered off in recent months with the influx of U.S. troops to the capital.

In the Mosul blast, police said explosives hidden in a parked car killed a civilian and wounded seven others across the city from the U.S. military base where Gates arrived.

The deadliest attack Wednesday was in Baqouba, 35 miles northeast of Baghdad, where a suicide car bomber targeted a bus station and killed five civilians, police said. At least 20 others were wounded.

In oil-rich Kirkuk, a parked car bomb killed three Kurdish soldiers in a convoy guarding a police chief traveling from Sulaimaniyah to the east, said police Brig. Anwar Qadir. At least 12 others were wounded — all members of the Kurdish peshmerga forces, he said.

Kirkuk is an ethnically mixed city near Iraq's semiautonomous Kurdish region, about 180 miles north of Baghdad. The area has seen a spike in violence in recent months, which U.S. military officials attribute to the northward flow of militants after American offensives around Baghdad.

Wednesday's attacks — although far fewer in number than months ago — underscored the danger that persists in Iraq.

The Iraqi military credited neighborhood watch-style groups with tamping down violence in Baghdad, and said the number of volunteers in such groups would quadruple next year.

"The reason behind the drop is the good performance of Iraqi security forces, support from Baghdad residents and the backing of U.S. troops," said Brig. Gen. Qassim al-Moussawi, the chief Iraqi military spokesman.

He said the number of volunteers in the Baghdad area would rise to 45,000 next year. There are 12,000 residents involved in so-called "Awakening" groups, patrolling their own enclaves for signs of militants, al-Moussawi said.

Also Wednesday, Iraqi police officers demonstrated outside a station in eastern Baghdad, calling for the release of a colleague detained by U.S. forces.

Police said the man, Hassan Jawad, was arrested by American soldiers who stormed the Al-Khansaa police station at 5:30 a.m. Wednesday. The U.S. military had no immediate comment on the incident.

About two dozen policemen chanted and unfurled banners in front of the station reading "U.S. forces should leave Iraq" and "Free Hassan."

"Is it right for a police station to be assaulted like this? We represent the country's authority, and taking one of us without reasons is wrong," said one of the protesters, who refused to give his name out of security concerns.

"This behavior makes us feel that we have no immunity — that U.S. soldiers can come another day to take any one of us," he said. "This is illegal and violates the country's sovereignty."

U.S. soldiers detained 10 suspected insurgents — some with ties to al-Qaida in Iraq — and captured weapons and propaganda materials, the military said.

One suspect thought to be involved in "terrorist media and propaganda operations" was detained in Baghdad, along with a "large amount" of propaganda materials, the military said.

North of Baghdad, troops captured a suspect accused of being involved in weapons manufacturing and car-bomb attacks in Tikrit. The military accused the suspect of having ties to other al-Qaida in Iraq weapons makers.

In Hadid, a village near Baqouba, 35 miles northeast of Baghdad, forces detained another suspect in propaganda and media operations. The detainee is allegedly tied to several al-Qaida in Iraq members recently detained or killed by coalition forces.

In Samarra — 60 miles north of Baghdad — and Mosul — 225 miles northwest of Baghdad — troops detained seven suspects as they targeted al-Qaida in Iraq members involved in roadside bomb attacks on troops and the support of foreign terrorists.