* Opposition demands government agree that Assad must go
* Government delegation rejects demands, defends "legitimate" leader
* UN's Ban calls for immediate humanitarian access
* Russia's Lavrov says outsiders must not meddle in Syria
* U.S. Secretary Kerry says "no way" Assad can keep power
By John Irish and Stephanie Nebehay
MONTREUX, Switzerland, Jan 22 (Reuters) - Syria's government and opposition, meeting for the first time at a U.N. peace conference, angrily spelled out their hostility on Wednesday as world powers also offered sharply divergent views on forcing out Bashar al-Assad.
Opposition leader Ahmed Jarba accused the president of Nazi-style war crimes and demanded the Syrian government delegation at the one-day meeting in Switzerland immediately sign up to an international plan for handing over power.
Syrian Foreign Minister Walid al-Moualem insisted Assad would not bow to outside demands and painted a graphic picture of "terrorist" rebel atrocities supported by Arab and Western states who back the opposition and were present in the room.
The United States and Russia, co-sponsors of the conference which U.N. officials hope will lead to negotiations in Geneva from Friday, also revealed their differences over Assad during a day of formal presentations at Montreux on Lake Geneva.
The talks reflect mounting global concern that a war which has killed over 130,000 and left millions homeless is spilling beyond Syria and fuelling sectarian militancy abroad. But there was little sign that any party was ready to make concessions.
U.S. Secretary of State John Kerry echoed the rebel view that there is "no way" Assad can stay under the terms of a 2012 international accord calling for an interim coalition. Russian Foreign Minister Sergei Lavrov said all sides must have a role and criticised "one-sided interpretations" of that 2012 pact.
Saudi Arabia, which backs the Sunni rebels, called for Iran and its Shi'ite Lebanese ally Hezbollah to withdraw forces from Syria. Iran, locked in a sectarian confrontation across the region, was absent, shunned by the opposition and the West for rejecting calls for a transitional government. Its president said Tehran's exclusion meant talks were unlikely to succeed.
The conference has raised no great expectations, particularly among Islamist rebels who have branded Western-backed opposition leaders as traitors for even agreeing to be in the same room as Assad's delegates.
United Nations Secretary-General Ban Ki-moon opened proceedings at Montreux by calling for immediate access for humanitarian aid convoys to areas under siege.
"After nearly three painful years of conflict and suffering in Syria, today is a day of fragile but real hope," Ban said, condemning a record of human rights abuses across the board.
He urged both sides in Syria to reach a full settlement based on the 2012 U.N. Geneva Communique, under which world powers called for a transitional government. "Great challenges lie ahead but they are not insurmountable," he added.
Many of the more than 40 government representatives echoed concerns about the human cost of the war and the dangers of escalation posed by heavily armed international militants.
But there was little sign of compromise on the central issue of whether Assad, who inherited power from his father 14 years ago, should make way for a government of national unity.
He himself says he could win re-election later this year and his fate has divided Moscow and Washington. Both endorse the conclusions of the 2012 meeting of world powers, known as Geneva 1, but differ on whether it means Assad must go now.
Opposition leader Jarba called for the government delegation to turn against their president before negotiations start: "We agree completely with Geneva 1," he said.
"We want to make sure we have a partner in this room that goes from being a Bashar al-Assad delegation to a free delegation so that all executive powers are transferred from Bashar al-Assad," he added.
"My question is clear. Do we have such a partner?"
Turning around the government's accusations that the rebels have fostered al Qaeda and other militants, Jarba said it was Assad's forces which, by targeting mainstream opposition groups, had created the conditions for al Qaeda to thrive.
Moualem exchanged sharp words with Ban as he spoke well beyond a 10-minute limit Ban had requested. The Syrian foreign minister called on foreign powers to stop "supporting terrorism" and to lift sanctions against Damascus. And he insisted that Assad's future was not up for discussion.
"We came here as representatives of the Syrian people and state and everybody should know that nobody in this world has the right to withdraw the legitimacy of a president or government ... other than the Syrians themselves," he said.
Lavrov repeated Moscow's opposition to "outside players" interfering in Syria's sovereign affairs and prejudging the outcome of talks on forming an interim government. He also said Iran - Assad's main foreign backer - should have a say.
Speaking of the Geneva Communique, he said: "The essence of this document is that mutual agreement between the government and opposition should decide the future of Syria."
Kerry also spoke of "mutual" agreement among Syrians, but in a sense that excluded Assad.
"We see only one option - negotiating a transition government born by mutual consent," he said. "That means that Bashar al-Assad will not be part of that transition government. There is no way, no way possible, that a man who has led a brutal response to his own people can regain legitimacy to govern."
A last-minute invitation from Ban to Iran was revoked after the Syrian opposition threatened to boycott the talks - a move that threatened to undermine months of U.S. and Western efforts to cajole Jarba's National Coalition into taking part.
Iranian President Hassan Rouhani said Tehran's exclusion made it unlikely the conference could succeed: "Because of the lack of influential players in the meeting, I doubt about the Geneva 2 meeting's success in fighting against terrorism ... and its ability to resolve the Syria crisis," Rouhani said.
"The Geneva 2 meeting has already failed without it even being started," he was quoted as saying by IRNA news agency - though he added he would be pleased if it did help bring peace.
WAR RAGES IN SYRIA
During the speeches in Montreux, the war went on in Syria.
The Syrian Observatory for Human Rights monitoring group reported clashes and air strikes around the country. Around Damascus, government artillery hit villages and rebels clashes with the army in the neighbourhood of Jobar on the northeast fringe of the capital, it said. Activists also reported clashes and in the central city of Hama, the southern province of Deraa - where the revolt began - and the northern city of Aleppo.
The release of thousands of photographs apparently showing prisoners tortured and killed by the government was cited by Jarba and Western ministers. The Syrian government rejected the report, which was backed by its enemy Qatar, as lacking objectivity and aimed at undermining peace efforts.
Discontent stretches back to the rule since 1970 of Assad's father, who took power in a military coup, but it boiled over in March 2011 as Syria's drought-hit economy struggled and the Arab Spring uprisings in Tunisia and Egypt inspired protests.
When those were crushed, the revolt became a war that has taken on an increasingly sectarian complexion, setting majority Sunnis against Assad's Alawite community, an offshoot of Shi'ite Islam. It has also drawn in rival powers with Saudi Arabia and Qatar backing the rebels and Iran standing by Assad.
Al Qaeda-linked militants and other Islamists have emerged as the most powerful forces on the rebel side, dampening Western appetite for direct intervention and sparking conflict among rival rebel formations. Iran and Hezbollah have helped Assad. And violence has spread, notably to Iraq and Lebanon.
For all the low expectations on Lake Geneva, millions of Syrians in refugee camps hope something will change: "Let them please find a solution for this problem," Mohammed from Homs said at a U.N. centre in Lebanon. "Let us go home."