CAIRO, Nov 27 (Reuters) - Opponents of President Mohamed Mursi clashed with Egyptian police on Tuesday as thousands of protesters stepped up pressure on the Islamist to scrap a decree they say threatens the nation with a new era of autocracy.

Police fired tear gas at stone-throwing youths in streets off Cairo's Tahrir Square, centre of the uprising that toppled Hosni Mubarak last year. A 52-year-old protester died after inhaling the gas, the second fatality since Mursi announced the decree expanding his powers and preventing court challenges to his decisions last week.

Tuesday's protest called by leftists, liberals and other groups marked a deepening of the worst crisis since the Muslim Brotherhood politician was elected in June, and exposed a divide between the newly-empowered Islamists and their opponents.

Some protesters have been camped out since Friday in the square, and violence has flared around the country, including in a town north of Cairo where a Muslim Brotherhood youth was killed in clashes on Sunday. Hundreds more have been injured.

Mursi's move has also provoked a rebellion by judges and battered confidence in an economy struggling to recover from two years of turmoil.

Opponents have accused Mursi of behaving like a modern-day pharaoh. The United States, a big benefactor to Egypt's military, has expressed its concerns about more turbulence in a country that has a peace treaty with Israel.

Mursi's administration has defended his decree as an effort to speed up reforms and complete a democratic transformation. Opponents say it shows he has dictatorial instincts.


"The people want to bring down the regime," protesters chanted, echoing slogans used in the anti-Mubarak uprising.

"We don't want a dictatorship again. The Mubarak regime was a dictatorship. We had a revolution to have justice and freedom," said 32-year-old Ahmed Husseini.

The protest was a show of strength by the non-Islamist opposition, whose fractious ranks have been pushed together by the crisis. Well-organised Islamists have consistently beaten more secular-minded parties at the ballot box in elections held since Mubarak was ousted in February, 2011.

Some scholars from the prestigious al-Azhar mosque and university joined Tuesday's protest, showing Mursi and his Brotherhood have alienated some more moderate Muslims.

MORE POWERS

Mursi formally quit the Brotherhood on taking office, saying he would be a president for all Egyptians, but is still a member of its Freedom and Justice Party.

The decree issued on Thursday expanded his powers and protected his decisions from judicial review until the election of a new parliament expected in the first half of 2013.

The New York-based Human Rights Watch said it gives Mursi more power than the military junta from which he assumed power.

U.N. Secretary-General Ban Ki-moon noted judges challenged the decree. However, he told Austria's Die Presse newspaper: "I have also noted that Mursi wants to resolve the problem in a dialogue. I will encourage him to continue to do so."

Trying to ease tensions with judges outraged at the step, Mursi has assured Egypt's highest judicial authority that elements of the decree giving his decisions immunity would apply only to matters of "sovereign" importance. Although that should limit it to issues such as a declaration of war, experts said there was room for a much broader interpretation.

In another step to avoid more confrontation, the Muslim Brotherhood cancelled plans for a rival mass protest in Cairo on Tuesday to support the decree. Violence has flared in the past when both sides have taken to the streets.

But there has been no retreat on other elements of the decree, including a stipulation that the Islamist-dominated body writing a new constitution be protected from legal challenge.

"We came here to reject dictatorship and tyranny," said 50-year-old Noha Abol Fotouh. "The decree must be cancelled and the constituent assembly should be reformed, all intellectuals have left it and now it is controlled by Islamists."

With its popular legitimacy undermined by the withdrawal of most of its non-Islamist members, the assembly faces a series of court cases from plaintiffs who claim it was formed illegally.

The new system of government to be laid out in the constitution is one of the issues at the heart of the crisis.

RAISING CONCERNS

"The president of the republic must put his delusions to one side and undertake the only step capable of defusing the crisis: cancelling the despotic declaration," liberal commentator and activist Amr Hamzawy wrote in his column in al-Watan newspaper.

Mursi issued the decree on Nov. 22, a day after his administration won international praise for brokering an end to eight days of violence between Israel and Hamas in Gaza.

White House spokesman Jay Carney said on Monday that Mursi had played "an important role" in the truce. "Separately we've raised concerns about some of the decisions and declarations that were made on Nov. 22," he said.

Mursi's decree was seen as targeting in part a legal establishment still largely unreformed from Mubarak's era, when the Brotherhood was outlawed.

Rulings from an array of courts this year have dealt a series of blows to the Brotherhood, leading to the dissolution of the first constitutional assembly and the lower house of parliament elected a year ago. The Brotherhood dominated both.

The judiciary blocked an attempt by Mursi to reconvene the Brotherhood-led parliament after his election victory. It also stood in the way of his attempt to sack the prosecutor general, a Mubarak hold over, in October.

In his decree, Mursi gave himself the power to sack that prosecutor and appoint a new one. In open defiance of Mursi, some judges are refusing to acknowledge that step.

Though both Islamists and their opponents broadly agree that the judiciary needs reform, Mursi's rivals oppose his methods. One presidential source said Mursi wanted to change the make-up of the Supreme Constitutional Court, the body whose ruling that parliament was void led the house being dissolved.

Mursi has repeatedly stated the decree will stay only until a new parliament is elected - something that can happen once the constitution is written and passed in a popular referendum. (Additional reporting by Tom Perry, Seham Eloraby, Marwa Awad and Yasmine Saleh in Cairo and Michael Shields in Vienna; Writing by Edmund Blair and Tom Perry; Editing by Anna Willard/David Stamp)