* Attack is deadliest beyond Sinai since Mursi's fall
* Army-backed government says it won't derail roadmap (Adds Interior Ministry, government comment, background)
By Yasmine Saleh
CAIRO, Dec 24 (Reuters) - A bomb blast tore through a police compound in Egypt's Nile Delta on Tuesday, killing 12 people and wounding 134 in one of the deadliest attacks since the army deposed Islamist President Mohamed Mursi in July.
The army-backed government vowed to fight "black terrorism", saying the blast an hour after midnight in the city of Mansoura north of Cairo would not derail a political transition plan whose next step is a January referendum on a new constitution.
With eight policemen among the dead, the blast pointed to the risk of militancy moving to the densely populated Nile Valley from the Sinai Peninsula, where attacks have killed some 200 members of the security forces since Mursi's downfall.
"We face an enemy that has no religion or nation," Interior Minister Mohamed Ibrahim, the survivor of an assassination attempt in September, said while inspecting the scene of the blast, an Interior Ministry statement said.
The military-backed presidency declared it a terrorist attack. "These type of operations only increase the state's determination to uproot terrorism across the country," it said in a statement published by state-run media.
Egypt has suffered the deadliest internal strife in its modern history since the army deposed Mursi, the nation's first freely elected leader, on July 3 after big protests against him.
The security forces killed hundreds of his supporters as part of a campaign to repress his Muslim Brotherhood, while lethal attacks on the security forces have become commonplace.
Tuesday's bombing prompted a cabinet statement declaring Mursi's Muslim Brotherhood a "terrorist organisation", though the bulletin carried by the state news agency did not explicitly accuse the group of staging the attack.
The Brotherhood condemned the blast, saying it was "an attack on the unity of the Egyptian people". Ibrahim said the police were investigating exactly how it had been staged.
State television showed the security building with shattered windows and one wall partially collapsed, and a bulldozer removing rubble in the street in front of it. A security source said the blast may have been caused by a car bomb.
Sinai has been the site of most major attacks since Mursi's ouster. A car bomb killed 10 soldiers in November and militants killed 24 policemen in an ambush in August.
Many of the Sinai attacks have been claimed by a group called Ansar Bayt al-Maqdis, "Supporters of Jerusalem". It also said it was behind the failed assassination attempt on Ibrahim in Cairo in September. Small-scale attacks occur almost daily.
There have also been lethal attacks on security forces in the Nile Delta, though nothing on the scale of Tuesday's blast.
Witnesses in Mansoura said many cars inside and outside the security compound were burned out and the entire city was in chaos as people hurried to hospitals to check on victims.
Egypt's Nile News TV cut into its late-night programming to urge people to go to hospitals to donate blood to the victims.
The government has accused the Brotherhood of turning to violence - charges the group denies. Last week, the public prosecutor ordered Mursi and other Brotherhood leaders to stand trial in two separate terrorist cases. (Additional reporting by Shadia Nasralla and Ali Abdelatti; Editing by Tom Perry and Alistair Lyon)