The brazen assault north of Cairo underscored the military's challenge to contain Sinai militants, who have killed more than 100 soldiers and police in a wave of attacks since the army ousted the Islamist president on July 3.
"Your brothers in Ansar Beit al-Maqdis, with the grace of God, were able to target the Daqhaleya police headquarters," the group said of yesterday's attack, in a statement posted on jihadist forums.
The group said the attack in the provincial capital Mansoura was carried out by a suicide bomber identified as "Abu Maryam."
Morsi and top Brotherhood leaders, imprisoned in a crackdown following his overthrow, are charged with colluding with militant groups to launch attacks.
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Ansar Beit al-Maqdis, whose name means "Partisans of Jerusalem" in English, had previously claimed credit for bombings in Sinai and the attempted assassination by a suicide car bomber of interior minister Mohamed Ibrahim in Cairo in September.
The group warned soldiers and police to abandon their posts "to preserve their religion and lives."
An interior ministry official told AFP today investigators were still trying to identify the suicide bomber through human remains found at the scene of the blast, which tore down part of the police headquarters facade.
The military has sent tanks and armour to the Sinai peninsula to crush the militants, with limited success so far.
The sparsely populated desert and mountain region has presented a challenge to the army, which is unaccustomed to fighting against a sustained militant campaign.
But the military says it has killed 184 "terrorists" in north Sinai, which borders the Palestinian Gaza Strip and Israel, since Morsi's overthrow.