In its statement posted on militant websites, Ansar denounced el-Said as "a renegade criminal" and warned of more attacks. "Expect the worst, as the time of punishment is near," it said.
Separately, the group said it blew up a natural gas pipeline on Monday night outside el-Arish, the provincial capital of North Sinai. Pipelines have repeatedly come under attacks since the 2011 anti-Mubarak uprising and the security vacuum that ensued.
Earlier Friday, two planted explosive devices detonated on a main highway on the western outskirts of Cairo, hitting a passing vehicle carrying riot police and wounding an officer, Interior Ministry spokesman Hani Abdel-Latif said.
Nearby, security forces shot dead a minibus driver at a police checkpoint in the Sixth of October suburb of the capital. The police said the driver was trying to ram the checkpoint, ignoring warning shots, the ministry said. The driver was killed, one passenger wounded, and three others were arrested it said, adding that the incident was under investigation.
Around the country, riot police firing tear gas clashed with hundreds of Morsi supporters protesting in Cairo, the Mediterranean coastal city of Alexandria and Fayoum, south of the capital.
In the fighting in Naam Square, in Cairo's Ain Shams district, the Interior Ministry said in a statement that clashes erupted between Brotherhood supporters and civilians and "automatic weapons and gunshots were used". The ministry said it deployed forces to separate between the two.
Hundreds of protesters were starting to set up a stage with loudspeakers in the square for a sit-in commemorating clashes a week ago in nearby Alf Maskan Square in which dozens were killed, one witness Ayat Soliman said. She said civilians attacked Friday's gathering with birdshot, sparking the violence.
AP correspondents Maamoun Youssef and Mariam Rizk contributed to this report from Cairo
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