A police brigadier-general has been killed and seven others were wounded in a series of explosions outside Cairo University in Giza governorate, according to state media.

Officials said the first two bombs went off seconds apart on Wednesday, and that the explosive devices had been concealed in a tree between two security posts.

A third explosion went off shortly afterwards near a police checkpoint. Egypt's interior ministry said that device was also planted in a tree.

The officer killed was identified as General Tarek al-Mergawi. According to AFP news agency, he was a chief detective who would have dressed in civilian clothes.

The explosions happened at the university's engineering faculty during clashes between students and security forces.

Police cordoned off the area, searching for other possible explosive devices. Helwan University, south of Cairo, was also evacuated and state TV reported arrests at Mansoura University, north of the capital.

Claim of responsibility

An armed group, known as Ajnad Misr, or "Egypt's Soldiers", claimed responsibility for the blasts in a statement it posted on their official Twitter account, saying the attacks were in retaliation for increased arrest campaigns targeting "our women".

The group vowed to continue the attacks "for as long as any of you remain in prison". Al Jazeera was unable to verify the authenticity of the account.

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"This last attack, near Ennahda Square, follows an increase in arrest campaigns targeting our women and girls and their abuse, and it is an honour to dedicate [the attacks] to them," the statement said.

"And we promise you that we will not find peace for as long as one you remains in captivity."

The group explained in the statement that the detonation of the third explosive device was delayed to avoid the injury of civilians who were nearby, while the first two were blown up after "leaders of criminal instruments" were seen coming close to the bombs' radius.

The country's security forces have been targeted in frequent attacks since a military coup toppled former President Mohamed Morsi last summer.

Though usually limited to the restive Sinai peninsula, attacks have spread to major urban areas in the mainland.

Two soldiers, including a brigadier-general, and five fighters were killed in clashes in Qalubiya province north of Cairo in March.

Supporters of Morsi have staged regular protests against the military-appointed government, which have often ended with bloodshed.

The government says fighters have killed almost 500 people, most of them police and soldiers since Morsi was ousted following protests against his rule.

Source: Al Jazeera and agencies