* Forces fire in air to scatter anti-government protesters
* Thousands of others chant support for army chief Sisi
* Two killed in Islamist demonstration in Minya
* Dozens arrested in second city Alexandria
* Many Egyptians tired of turmoil, want Sisi as president
By Sameh Bardisi and Maggie Fick
CAIRO, Jan 25 (Reuters) - Security forces fired into the air to disperse anti-government protesters on Saturday as thousands rallied in support of the army-led authorities, underlining Egypt's volatile political fissures three years after the fall of autocrat Hosni Mubarak.
Instead of commemorating Mubarak's overthrow in a popular uprising, a large number of Egyptians gathered in Cairo's Tahrir Square to pledge their support for the army chief who ousted the country's first freely-elected president last year.
The chanting for General Abdel Fatah al-Sisi underscored the prevailing desire for a decisive military man they count on to end the political turmoil that has gripped Egypt since the 2011 Arab Spring revolution and crippled the economy.
But an end to street violence seemed nowhere in sight with the sound of tear gas canisters being fired echoing through downtown Cairo as police confronted anti-government protesters.
Sisi toppled President Mohamed Mursi of the Muslim Brotherhood in July after mass protests against what critics called his mismanagement and increasingly arbitrary rule, triggering a confrontation with the veteran Islamist movement that has hit investment and tourism hard.
The general, who served as head of military intelligence under Mubarak, is expected to announce his candidacy for the presidency soon and likely to win by a landslide in elections, expected within six months.
Several leading politicians have indicated they would not run for president if Sisi does, highlighting his dominance and the barren political landscape that has emerged since Mubarak's fall. The most vocal critics of the new order - the Brotherhood - have been driven underground.
Tensions have been smouldering anew since a wave of deadly bombings killed six people in Cairo on Friday. An al Qaeda-inspired group, based in the lawless Sinai Peninsula, claimed responsibility, according to the SITE monitoring organisation.
Early on Saturday a bomb exploded near a Cairo police academy. No one was hurt, said the Interior Ministry.
In the southern town of Minya, two people were killed in clashes between Mursi supporters and security forces, said Brigadier General Hisham Nasr, director of criminal investigations in the regional police department.
In Tahrir, the symbolic heart of the 2011 uprising, the mood on Saturday felt more like a campaign rally for Sisi than a commemoration of the 18-day revolt that Egyptians at the time hoped would bring democratic, civilian government.
Huge banners, posters and T-shirts displayed images of Sisi in his trademark dark sunglasses at Saturday's rally. Several hundred people chanted slogans in support of the general.
A woman named Heba dismissed the 2011 uprising and said the important revolution came when Egyptians held mass protests that led to the army takeover last July. "I'm here to support Sisi," she said.
TEAR GAS AND BIRDSHOT
Others didn't have the chance to express their views. Police fired live rounds in the air to disperse about 1,000 anti-government protesters in Cairo's Mohandiseen district and at two other marches in downtown.
Some of the demonstrators were supporters of the Brotherhood, while others were liberal activists. Witnesses said police also fired tear gas and birdshot at a crowds of activists moving toward Tahrir for an anti-government rally.
Hisham Sadiq, a university student, said he was protesting against "military rule and the thugs of the Interior Ministry".
At one rally, the crowd yelled "the people want the downfall of the regime!" - a common chant during the 18-day revolt that ousted Mubarak - before running from tear gas.
Dozens of anti-government protesters were arrested in Egypt's second city Alexandria, security sources said.
When he removed Mursi, Sisi promised a political roadmap that would lead to free and fair elections.
But the Muslim Brotherhood says Sisi and his allies in the government have blood on their hands and accuse them of undermining democratic gains made since Mubarak's downfall.
Security forces have killed up to 1,000 Muslim Brotherhood supporters and put the movement's top leaders in jail. The Brotherhood, which renounced violence in the 1970s, has been declared a terrorist group.
But the tough measures have failed to stabilise Egypt, which is of great strategic importance because of its peace treaty with Israel and control over the Suez Canal.
Islamist militants based in the Sinai Peninsula have stepped up attacks against security forces since Sisi toppled Mursi. Hundreds have been killed.
Investigators were also trying to determine the cause of a military helicopter crash in Sinai, the army spokesman said.
The security crackdown has been extended to secular-minded liberals, including ones who played a key role in the 2011 uprising. Human rights groups have accused the Egyptian authorities of quashing dissent and using excessive force, calling state violence since Mursi's ouster unprecedented.
Still, many Egyptians choose to look the other way and extend their full support to Sisi.
Friday's bombings stirred more public anger against the Brotherhood and calls for a harsher campaign against them.
"We are here to support Sisi," said a man in Tahrir who only gave his first name, Mahmoud. "Sisi is going to save the country," said his wife.