The attacks followed what was already the deadliest day in Iraq in a year -- on Thursday, two suicide bombers killed more than 80 people in Baghdad and north of the city.
The wave of violence, which has targeted Shiites, is raising fears of renewed sectarian violence as the U.S. begins to reduce its troop presence in advance of a 2011 withdrawal.
Friday's attack occurred in Baghdad's Kadhimiya neighborhood, near the mosque next to the tomb of Imam Mousa al-Khadum, one of the 12 imams of Shiite Islam. The death toll could be as high as 75, the Washington Post reported.
An interior ministry official told the New York Times that most of the dead were Iranians making pilgrimages to the shrine. The streets around the shrine had already been hit by two suicide bombers this year, according to the New York Times.
Thursday's bombing in the Diyala province, about 50 miles north of Baghdad, also targeted Iranian Shiite pilgrims. The suicide bomber struck a restaurant where busloads of pilgrims had stopped for lunch, and killed at least 47.
Also on Thursday, a female suicide bomber blew herself up in a crowd of Iraqis waiting in a food aid line in a Shiite district of central Baghdad, killing at least 28.
The attacks are raising fears that insurgent groups, which had lost support and backing in the last two years, are gaining renewed strength.
"The government was treating the situation like they'd won a victory," Sheik Jalal al-Din Saghir, a Shiite member of Parliament, told the Times. "They relaxed. We can't ignore that there were security successes, but that doesn't mean the story is finished."
Last month, the umbrella group the Islamic State of Iraq, which includes the Sunni al-Qaida in Iraq, announced it would begin a wave of violence called "The Good Harvest."
Since then the pace of bombings in Iraq has increased.
On Thursday the Iraqi government announced that it had captured a major leader of the group, Abu Omar al-Baghdadi. But the U.S. has yet to confirm the arrest, and previous such claims have turned out to be wrong. In the past U.S. officials have even suggested that al-Baghdadi might be a fictional character developed to put an Iraqi face on a group made up mostly of foreign fighters.
Mariam al-Rayyis, a senior advisor to Prime Minister Nouri al-Maliki, told the Post that the recent uptick in violence has not made the Maliki government consider altering the U.S. withdrawal timetable negotiated last fall.
U.S. troops "want to pack their bags and leave as soon as possible," al-Rayyis said.
But some Iraqis believe that moves made by the U.S. in advance of the withdrawal have inflamed the violence. The U.S. is emptying its detention centers, and not all of the inmates have been transferred to Iraqi prisons.
"This is the ugly work of al-Qaida members who are being released from American prisons," Muhammad Abdulla, 30, a bystander to Friday's bombing, told the Post. "The U.S. is doing this on purpose to find an excuse to extend its presence in Iraq."