Bombs blast at police compound kills at least 13 in Egypt Scene of the explosion in Mansoura at a security headquarters At least 13 people, most of them policemen, were killed and more than 100 injured when a massive bomb destroyed a security headquarters in the northern Egyptian town of Mansoura in the early hours of Tuesday morning. The blast, the largest attack outside the troubled Sinai peninsula, marks a significant escalation in violence in the aftermath of the popularly backed coup which ousted Mohamed Morsi, the elected Islamist president in July. It comes three weeks before a referendum on a new constitution intended to turn the page on Mr Morsi's era and lay the ground for a new political order sidelining his Muslim Brotherhood group. It was not clear who was responsible for the attack. The bombing is likely to hand a publicity victory to the military-backed authorities as they mobilise support for a "yes" vote in the referendum, appealing to outraged citizens longing for stability and fed up with political upheaval since the 2011 revolution which toppled Hosni Mubarak as president. Already officials have described the Brotherhood group, which dominated politics until July, as "terrorist." The interim authorities were quick to pin the blame on the Islamists with a cabinet spokesman saying the Brotherhood has "shown its ugly face as a terrorist organisation, shedding blood and messing with Egypt's security." He said Egyptians will not be deflected from pursuing their new political road map, which includes the constitutional referendum to be followed by elections for a new parliament and president. The Brotherhood and a group of small Islamist parties allied to it had called last week for a boycott of the referendum. An internet statement from the Brotherhood, which has been banned and hunted for months with most of its leaders in prison, condemned the Mansoura attack and accused the authorities and "corrupt businessmen" of standing behind it. On Tuesday morning, angry commentary on Egyptian state and private television channels held the Brotherhood responsible for the carnage, and viewers who called in urged the government to enact tougher measures against the group. Footage of the wrecked police building in Mansroua, surrounded by rubble, showed a man carrying a large placard with a picture of Mohamed Badie, the jailed leader of the Muslim Brotherhood hanging from a noose and dressed in the red suit of prisoners on death row. Mohamed Ibrahim, the interior minister, was quoted by official media as saying that Egypt "faced an enemy which recognises neither religion nor country and which seeks to destroy the state." The bombing may, for now, quiet international condemnation of Egyptian authorities for the sentencing on Sunday of three renowned democracy activists, to three years in prison on charges of taking part in unauthorised peaceful protests. William Hague, UK foreign minister, two days after criticising Cairo authorities for the imprisonments, issued a statement on Tuesday strongly condemning the bombing. "The Egyptian people are determined to build a stable and prosperous country for themselves," the statement said. We stand with them at this difficult time. No one has claimed responsibility for the explosion, but an al-Qaeda inspired group in Sinai responsible for recent bombings and shootings against police and the military had issued fresh threats on Monday. Ansar Beit al-Maqdis vowed to carry out more attacks on soldiers and policemen saying that it considers troops to be infidels because they answer to the secular military-backed authorities. In a statement on jihadi websites, the group urged troops to repent from serving "this infidel bastion that is at war with God and his prophet." The militant group was behind a failed attempt in September to assassinate Mr Ibrahim by a suicide car bomb, detonated as his convoy drove through a Cairo street near his home. Most of its attacks have been in the Sinai peninsula, with the occasional strike outside its main area of operations. An Egyptian military spokesman, Ahmed Mohamed Ali, said on Monday that an army crackdown in the Sinai has led to 184 militants being killed and 803 others arrested. He said roughly a quarter of those killed and arrested were foreign fighters, but gave no details of their nationalities. Additional reporting from Borzou Daragahi