BAGHDAD — A string of at least 20 explosions shook the Iraqi capital Tuesday night, most of them in Shiite neighborhoods, authorities said.

The rare coordinated attack, which included car bombings, roadside bombings and mortar attacks, prompted officials to impose a curfew in the capital shortly after 8 p.m.

The blasts killed at least 63 people and wounded 285, Iraqi police officials said, citing early reports from the field. The Associated Press reported 76 killed, citing unnamed Iraqi offiicals.

Teams of U.S. soldiers were dispatched to some of the blast sites to assist Iraqi security forces, U.S. military spokesman Lt. Col. Eric Bloom said, adding that American military radars had picked up between 13 and 17 explosions Tuesday night, none thought to be the result of a mortar or rocket attack. Bloom said the attacks appeared to be the work of the Sunni insurgent group al-Qaida in Iraq.

“This seems to be typical AQI tactics,” he said in an e-mail.

American soldiers have become a rare sight in Baghdad since June 2009, the deadline the Iraqi government set for their nominal departure from cities as part of a bilateral agreement signed the year before.

Coming two days after an attack on a Baghdad Catholic church during Sunday Mass in which 58 people were killed, the newest wave of violence infuriated Iraqis. Many blamed the worsening security situation on their elected officials, who have been unable to form a government since parliamentary elections March 7.

“There is no government,” Baghdad resident Hamid Ahmed al-Azawi, 51, said. Referring to Iraq’s political leaders, many of whom live inside the heavily fortified Green Zone, he added: “If the Americans leave tomorrow, we will assemble a team of 500 armed men to topple the Green Zone.

“How much longer are the Americans going to protect them?”

Lawmakers are at loggerheads over who is entitled to become prime minister and whether the post should be weakened. As the blasts began thundering through the city shortly after sunset, traffic thinned, ambulances wailed and U.S. helicopters circled overhead as this violence-weary capital returned to a familiar defensive posture.