(Repeats for additional subscribers)
* Government blames former president's Muslim Brotherhood
* Prime Minister declares Brotherhood a "terrorist" group
By Yasmine Saleh
CAIRO, Dec 24 (Reuters) - At least 11 people were killed and more than 100 wounded in a massive explosion at a security compound in Egypt's Nile Delta city of Mansoura in Dakahlyia province on Tuesday, a month before a vote on a new constitution key to transition from military-backed rule.
Cabinet spokesman Sherief Shawki blamed the blast on the Muslim Brotherhood group of former Islamist President Mohamed Mursi, and said Prime Minister Hazem el-Beblawi had officially declared the group a terrorist organisation.
A security source said the blast may have been caused by a car bomb, but it was not clear if it was suicide attack or not.
State TV said at least 11 people were killed and more than 100 hurt. It said the attack was the worst in the city's history and two senior security officials were among the wounded.
Another security source it was still unclear what caused the explosion, "but it seems to be a big one that led to the collapse of parts of the security building".
Suicide attacks on soldiers and policemen have sharply risen in Egypt since the army ousted Mursi in July amid mass unrest against his rule.
In overthrowing Mursi, the army set out a political roadmap supposed to start with a mid-January vote on a new constitution and to be followed by presidential and parliamentary votes.
The state-run Middle East News Agency quoted Beblawi as saying that those responsible for blast "will not escape punishment".
Brotherhood officials, most of whom are on the run or out of the country, could not be immediately reached for comment on the government charges.
CHAOS, BURNED CARS
Witnesses said many cars inside and outside the security compound were burned out and the entire city was in chaos as people were hurrying 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.
Most bomb attacks since Mursi's overthrow have occurred in the Sinai region, which borders Israel and the Palestinian Gaza Strip. Around 200 soldiers have died in Sinai since July.
Interior Minister Mohamed Ibrahim survived a suicide car bomb attack targeting his convoy near his home in Cairo last September.
Egyptian media and many army and state officials have blamed all the violence on Mursi's Brotherhood, the state's oldest and most organised Islamist group, which has won all five elections since a 2011 uprising that ousted longtime President Hosni Mubarak.
Mursi's dramatic exit triggered a wave of violence that started with police attacks on his supporters' two main camps in Cairo on Aug. 14 in which hundreds were killed.
That violence prompted a wave of attacks on churches and police stations. Thousands of Islamist activists have been arrested in the unrest and around 100 policemen killed. (Reporting by Yasmine Saleh; Editing by David Brunnstrom)