An Iraqi policeman inspects the remains of a truck used in a bomb attack in Kirkuk city, northern Iraq on 07 June 2014. (Khalil Al-A'nei/EPA)

A series of car bombs exploded across Iraq’s capital Saturday night, killing at least 52 people in a day of violence that saw militants storm a university in the country’s restive Anbar province and take dozens hostage, authorities said.

The attacks in Baghdad largely focused on Shiite neighborhoods, underscoring the sectarian violence striking at Iraq years after a similar wave nearly tore the country apart after the U.S.-led invasion that toppled Saddam Hussein. With U.S. troops gone, Iraq finds itself fighting on fronts across the country, as separate clashes in the northern city of Mosul left 21 police officers and 38 Sunni militants dead, officials said.

The first Baghdad attack took place Saturday night in the capital’s western Baiya district, killing nine people, police said. Later, seven car bombs in different parts of the city killed at least 41 people, police said, adding that a roadside bomb in western Baghdad also killed two people.

The day began with militants killing three police officers on guard at the gates of Anbar University, a police and a military official said. Islamic extremists and other anti-government militias have held parts of Anbar’s nearby provincial capital of Ramadi and the city of Fallujah since December amid rising tensions between Sunni Muslims and the Shiite-led government in Baghdad.

The gunmen detained dozens of students inside a university dorm, the officials said. Sabah Karhout, the head of Anbar’s provincial council, said hundreds of students were in the university compound when the attack started. Anbar University says it has more than 10,000 students, making it one of the country’s largest.

Ahmed al-Mehamdi, a student who was taken hostage, said he awoke to the crackle of gunfire, looked out the window and saw armed men dressed in black running across the campus. Minutes later, the gunmen entered the dormitory and ordered everybody to stay in their rooms while taking others away, he said.

The Shiite students at the school were terrified, Mehamdi said, as the gunmen identified themselves as belonging to an al-Qaeda splinter group known as the Islamic State of Iraq and Syria. The Sunni terror group, fighting in Syria with other rebels trying to topple President Bashar al-Assad, is known for massive, bloody attacks in Iraq as well often targeting Shiites that they view as heretics.

The Islamic State did not immediately claim the attack on the school.

Several hours later, gunmen left the university under unclear circumstances.

“Not a single student or a university staff member was hurt during the raid,” Karhout said.