BAGHDAD — Bombers and gunmen launched an apparently coordinated string of attacks today against Iraqi government forces, killing at least 50 people a day after the number of U.S. troops fell below 50,000 for the first time since the start of the war.

Insurgents have been stepping up their attacks on Iraq's security forces in recent months as the U.S. has trimmed its military presence in the country. More than half of those killed today — 27 — were Iraqi soldiers and policemen.

There were no claims of responsibility for the spate of attacks. But their scale and reach, from one end of the country to the other, underscored insurgent efforts to prove their might against security forces and political leaders who are charged with the day-to-day running and stability of Iraq.

The deadliest attack came in Kut, 100 miles southeast of Baghdad, where a suicide bomber blew up a car inside a security barrier between a police station and the provincial government's headquarters.

Police and hospital officials said 19 people were killed, 15 of them policemen. An estimated 90 people were wounded.