(Adds anti-Bush protests, Cuba foreign minister walking out)

UNITED NATIONS, Sept 25 (Reuters) - U.S. President George W. Bush on Tuesday rallied fellow U.N. members to what he called a mission of liberation and named Belarus, Syria, Iran and North Korea as "brutal regimes" that deny people their rights.

With national representatives seated before him on the opening day of the U.N. General Assembly, Bush also scolded the governments of Myanmar, Zimbabwe and Cuba as he called for the spread of democracy, a consistent theme of his U.N. speeches.

"This great institution must work for great purposes: to free people from tyranny and violence, hunger and diseases, illiteracy and ignorance and poverty and despair. Every member of the United Nations must join in this mission of liberation," he said.

Bush said Americans were "outraged" over human rights abuses in Myanmar and announced new U.S. sanctions on its military rulers who are facing the biggest anti-government protests in two decades.

He criticized the Zimbabwe government headed by President Robert Mugabe as "tyrannical" and an "assault on its people."

"The United Nations must insist on change in Harare and must insist for the freedom of the people of Zimbabwe," Bush said.

Critics accuse Mugabe of sending Zimbabwe's once-thriving economy to a crisis of widespread food shortages and soaring inflation. Mugabe accuses Western countries of sabotaging the economy as punishment for his seizure of white-owned farms to resettle landless blacks.

Alluding to Cuban leader Fidel Castro, who has been ill, Bush said his rule of the island was "nearing its end" and said free speech and elections should follow a transition in power.

The comment prompted Cuban Foreign Minister Felipe Perez Roque to walk out of the U.N. General Assembly hall in protest. Cuba's U.N. mission called Bush's speech "arrogant."

PROTESTERS OUTSIDE

Addressing the crisis in Sudan's western Darfur region, which has been ravaged by violence, Bush said the United Nations must follow through on a pledge to deploy peacekeeping forces.

"In Belarus, North Korea, Syria and Iran, brutal regimes deny their people the fundamental rights enshrined in the Universal Declaration (of Human Rights)," he added.

Bush praised some countries as having "taken strides toward liberty, including Ukraine and Georgia and Kyrgyzstan and Mauritania and Liberia, Sierra Leone and Morocco."

Bush repeated U.S. criticism of the U.N.'s Human Rights Council, which he said "has been silent on repression by regimes from Havana to Caracas to Pyongyang and Tehran while focusing its criticism excessively on Israel."

U.S. foes see Bush's "freedom agenda" as a way to bully countries which the Bush administration opposes.

They say that while Washington is pointing the finger at others, it has faced widespread condemnation for its treatment of prisoners in Iraq and Afghanistan and terrorism suspects at the U.S. military prison at Guantanamo Bay, Cuba.

Outside the United Nations, around 700 protesters chanted, "Bush and Cheney out the door, stop the torture, stop the wars." Many wore orange to symbolize action, and some carried coffins to highlight opposition to the Iraq war.

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