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.

Fireworks, not bombings, colored the Baghdad skyline late Monday, and thousands attended a party in a park where singers performed patriotic songs.

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