By Maggie Fick and Omar Fahmy

CAIRO, Nov 18 (Reuters) - About 1,000 people marched in Cairo's Tahrir Square on Monday night to condemn the actions of Egyptian security forces and some voiced rare criticism of the army chief, raising tension on the eve of planned mass protests.

Large numbers of demonstrators were expected to turn out on Tuesday, extending turmoil that has dogged Egypt since the army ousted elected Islamist President Mohamed Mursi and has wrecked investment and tourism in the major, U.S.-allied Arab state.

Monday's rally began in the afternoon to commemorate people killed in clashes with security forces two years ago, then turned into chanting against General Abdel Fattah al-Sisi, whose forces toppled Mursi in July.

Army and police who had been stationed by armoured personnel carriers after sealing off Tahrir melted away by the time the protesters arrived at the square, the heart of the 2011 popular uprising that ejected autocrat Hosni Mubarak.

"A word in your ear, Sisi, don't dream of being my president!" the protesters yelled. It was not clear why army and police units made no attempt to stop the demonstrators.

Sisi has become wildly popular in Egypt since Mursi's exit, and many believe he would win if he runs for president in elections expected next year.

But some Egyptians are opposed both to Mursi's Muslim Brotherhood and the current military-steered government, and they dominated the Tahrir Square rally.

Tuesday's expected mass protests will commemorate the second anniversary of the deaths of 42 people opposed to the military council ruling Egypt at the time in clashes with security forces on Mohamed Mahmoud Street, near Tahrir.

The army toppled Mursi a year after he took over from the generals who have dominated Egyptfor decades.

A major element in the instability plaguing Egypt since then has been militants based in the volatile Sinai who have stepped up violence since July, killing security forces almost daily in the peninsula near Israel.

Militant operations in Cairo have stirred concern that an Islamist insurgency could take hold beyond the Sinai in the Arab world's largest country, which contains the Suez Canal, the vital, quickest sea route between Asia and Europe.

The army says a political roadmap will lead to free and fair elections and stability to Egypt, where the revolt that brought down Mubarak in February 2011 raised prospects for democracy after decades of authoritarian rule.

HIGH-PROFILE SHOOTING

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."

A small minority of hardcore activists are using the anniversary of the November 2011 clashes on Mohamed Mahmoud Street to highlight the actions of security forces who have acted mostly with impunity in political upheaval since 2011.

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 organise the protests that begin this evening.

Interim Prime Minister Hazem el-Beblawi visited Tahrir earlier in the day to lay the cornerstone of a planned memorial that is disputed.

The government says the monument will honour the "martyrs" not only of the 2011 anti-Mubarak 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.

Some protesters sprayed graffiti on the monument on Monday.

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)