BAGHDAD — Iraqi forces assumed formal control of Baghdad and other cities today after American troops handed over security in urban areas in a defining step toward ending the U. S. combat role in the country.

A countdown clock broadcast on Iraqi TV ticked to zero as the midnight deadline passed for U. S. combat troops to finish their pullback to bases outside cities.

“The withdrawal of American troops is completed now from all cities after everything they sacrificed for the sake of security,” said Sadiq al-Rikabi, a senior adviser to Prime Minister Nouri al-Maliki. “We are now celebrating the restoration of sovereignty.”

The Pentagon did not offer any comment to mark the passing of the deadline.

Fireworks, not bombings, colored the Baghdad skyline late Monday, and thousands attended a party in a park where singers performed patriotic songs. Loudspeakers at police stations and military checkpoints played recordings of similar tunes throughout the day, as Iraqi military vehicles decorated with flowers and national flags patrolled the capital.

“All of us are happy — Shiites, Sunnis and Kurds on this day,” Waleed al-Bahadili said as he celebrated at the park. “The Americans harmed and insulted us too much.”

Al-Maliki declared a public holiday and proclaimed June 30 as “National Sovereignty Day.”

Midnight’s handover to Iraqi forces filled many citizens with pride but also trepidation that government forces are not ready and that violence will rise. Shiites fear more bombings by Sunni militants; Sunnis fear that the Shiite-dominated Iraqi security forces will give them little protection.

If the Iraqis can hold down violence in the coming months, they will show the country is finally on the road to stability. If they fail, it will pose a challenge to President Obama’s pledge to end an unpopular war that has claimed the lives of more than 4,300 U. S. troops and tens of thousands of Iraqis.

The gathering in the Baghdad park was unprecedented in size for such a postwar event in a city where people tend to avoid large gatherings for fear of suicide bombers. They ignored an appeal by Vice President Tariq al-Hashemi to stay away from crowded places during the U. S. pullback. More than 250 people have been killed in bombings over the past 10 days.

In a ceremony rich with symbolism, Maj. Gen. Daniel Bolger, the top U. S. military commander in Baghdad, gave his Iraqi counterpart the keys to the former defense ministry building, which had served as a joint base.

Despite today’s formal pullback, some U. S. troops will remain in the cities to train and advise Iraqi forces. U. S. troops will return to the cities only if asked. The U. S. military will continue combat operations in rural areas and near the border, but only with the Iraqi government’s permission.

The U. S. military, meanwhile, said an American soldier was killed Monday in combat in Iraq.

At least 4,319 members of the U. S. military have died in the Iraq war since it began in March 2003, according to an Associated Press count.

The U. S. has not said how many troops will be in the cities in advisory roles, but the vast majority of the more than 130,000 U. S. forces remaining in the country will be in large bases scattered outside cities.

Some fears have arisen that the 650,000-member Iraqi military is not ready to maintain stability and deal with a stubborn insurgency.

Privately, many U. S. officers worry the Iraqis will be overwhelmed if violence surges, having relied for years on the Americans for nearly everything.

“We think they are ready,” U. S. Ambassador Christopher Hill told the AP in an interview Monday.

Gen. David Petraeus, commander of U. S. troops in the Middle East, expressed concern about the spate of high-profile bombings but said the average daily number of attacks remained low at 10 to 15, compared with 160 in June 2007.

“While certainly there will be challenges—there are many difficult political issues, social issues, governmental development issues—we feel confident in the Iraqi security forces continuing the process of taking over the security tasks in their own country,” Petraeus said after meeting with Egyptian President Hosni Mubarak in Cairo.