By Hadeel Al Shalchi and Maggie Fick
CAIRO (Reuters) - Egyptian authorities blamed Islamist militants for the high-profile shooting death of an Interior Ministry lieutenant colonel, heightening security fears on Monday in a country on the edge with mass protests looming.
Police and army sealed off Cairo's Tahrir Square on the eve of a planned major rally to commemorate bloody clashes two years ago between demonstrators and security forces on nearby Mohamed Mahmoud street. Security forces killed 42 people opposed to the military council ruling Egypt at the time.
The army toppled elected president Mohamed Mursi of the Muslim Brotherhood on July 3, a year after he took over from the generals. Since then, Islamist militants based in the Sinai Peninsula have stepped up attacks on security forces in Egypt, one of the United States' most important regional allies.
Militant operations in Cairo have raised concerns that an Islamist insurgency could take hold beyond the Sinai.
On Sunday, Interior Ministry Lieutenant-Colonel Mohamed Mabruk was shot dead outside his home in Cairo's Nasr City district, the highest-profile killing in Cairo since Mursi's overthrow following mass unrest against his rule.
The attack occurred three days after a three-month curfew and state of emergency were lifted.
Mabruk, who was gunned down by masked men, was in charge of monitoring the Muslim Brotherhood movement in the Interior Ministry's National Security division.
"Our investigations point to Islamic jihadists carrying out this assassination with political motives," a security official told Reuters. "They were taking revenge on Mabruk because he took care of a very high-profile file."
Protests by Egyptians opposed both to military-dominated rule and the Brotherhood began in downtown Cairo on Monday night. One banner read, "Down with the dogs of the Interior Ministry". Another: "The military killed our children".
The army says a political roadmap will lead to free and fair elections and stability to Egypt, where the popular uprising which toppled Mubarak in February 2011 raised prospects for democracy after decades of authoritarian rule.
Since Mursi's exit, army chief General Abdel Fattah al-Sisi has become a wildly popular figure in Egypt with state media whipping up public sentiment against the Brotherhood.
A small minority of hardcore activists from the 2011 uprising are questioning this official narrative.
They are using the anniversary of the November 2011 clashes on Mohamed Mahmoud Street to draw attention to the actions of security forces, who have acted mostly with impunity in nearly three years of political upheaval.
In unusual defiance of the army, some activists wrote on social media about their desire to overthrow what they call the new "military junta", a reference to the interim government installed by the army after Mursi's removal.
"Our goal must be summed up as the downfall of the ruling military junta," an activist said on a Facebook page set up to organize the protests that begin this evening.
Interim Prime Minister Hazem el-Beblawi visited Tahrir to lay the cornerstone of a planned memorial that is disputed.
The government says the monument will honor the "martyrs" not only of the 2011 uprising, but also of what it calls the "June 30 revolution," referring to the date of the mass disturbances that precipitated Mursi's ouster. Activists say it insults the memory of protesters killed by security forces.
"Creating a memorial to say we respect the souls of the martyrs without ensuring accountability is basically a joke," said Ziad Abdel Tawab of the Cairo Institute for Human Rights Studies.
One of the protesters in downtown Cairo on Monday night, Ahmed Abdel Hamid, held up a poster of a friend he said was killed by security forces during the Mohamed Mahmoud clashes.
"I'm a son of the revolution and I came out on June 30 to demonstrate against the Brotherhood. But I don't accept what's happening in the prisons now. I don't accept the arbitrary arrests."
Aside from the Mohamed Mahmoud protests, an Islamist coalition that includes the Brotherhood has called on backers to take to the streets, and Tuesday is Sisi's birthday, raising the possibility that his own followers will demonstrate too.
(Writing by Michael Georgy; Editing by Mark Heinrich)