Meanwhile, authorities arrested another top figure from former President Mohammed Morsi's Muslim Brotherhood group, whose supporters have held near-daily protests against the Islamist's ouster in a July 3 coup.
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In a sign authorities are responding to an easing of those pro-Morsi rallies, the government once again shortened a military-imposed curfew by two hours, making the evening lock-up in nearly a dozen Egyptian provinces only seven hours.
However, the full 7 pm to 6 am curfew will remain in place for Fridays, when supporters of Morsi usually organize large rallies. Hundreds of his supporters, including leading Brotherhood members, have been arrested in a crackdown on the group.
Violence in the Sinai peninsula however has only worsened since Morsi's overthrow. The militant arrested Saturday is suspected of leading an al-Qaeda-linked group in an ambush where 25 off-duty policemen were lined up and shot last week, security officials said. The attack was one of the area's worst militant strike on security forces.
The militant, Adel Mohammed, also known as Adel Habara according to one official, has already been sentenced to death in absentia for killing soldiers in the Nile Delta last year.
Habara's arrest could potentially undermine militant activities in the area, where over two dozen security men have been killed alone since July. The security official said two other suspects were arrested along with Habara.
Authorities have been engaged in a long-running battle against militants in the northern half of the strategic region, which borders the Gaza Strip and Israel.
Earlier Saturday, security officials said 31 suspected militants have been arrested since Thursday, including three suspected of involvement in last week's attack and two caught seeking treatment for wounds sustained in clashes with police.
In other violence nearby, a riot police officer was shot in the chest Friday while on patrol in the city of El-Arish, the capital of North Sinai governorate, the Interior Ministry said Saturday.
According to one security official, four militants have also been killed since Thursday. All officials spoke on condition of anonymity because they were not authorized to speak to the media.
Since the ouster of Morsi, an Islamist, his supporters have been organizing regular protests calling for his return. Several thousand took to the streets on Friday.
Saturday's arrest in the Mediterranean city of Alexandria of senior Brotherhood leader Sobhi Saleh was made on charges of inciting violence and disrupting public order, according to security officials.
The son of another Brotherhood figure, Saad Emara, was also detained and ordered held for 15 days, pending investigations for charges of inciting violence.
Also on Saturday, Health Ministry spokesman Khaled el-Khateeb raised the death toll in violence during Friday's pro-Morsi protests in several cities to eight. El-Khateeb said 221 were also injured -- most of them in clashes between Morsi supporters and local residents.
Those killed on Friday included one police officer and one civilian shot dead in a drive-by shooting targeting a police station in an upscale neighborhood of Cairo.
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