* Opposition demands government agree Assad must go
* Syrian govt clashes with chair, seeks more time to talk
* UN's Ban calls for immediate humanitarian access
* Russia's Lavrov says outsiders must not meddle in Syria
* U.S. secretary Kerry said Assad can have no role in transition
By John Irish and Stephanie Nebehay
MONTREUX, Switzerland, Jan 22 (Reuters) - Syria's government and opposition, meeting face to face for the first time at a U.N. peace conference, angrily spelled out their hostility on Wednesday as world powers also restated contrasting views on the future of President Bashar al-Assad.
Opposition leader Ahmed Jarba accused Assad of war crimes that recalled Nazi Germany and demanded the Syrian government delegation at the one-day meeting in Switzerland immediately sign up to an international plan for a transition of 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 can launch further negotiations at Geneva, also revealed their differences over Assad in speeches that began what will be a day of formal presentations.
United Nations Secretary-General Ban Ki-moon, who exchanged sharp words with Moualem when the Syrian minister spoke for more than three times the 10-minute limit Ban had set, opened proceedings at Montreux on Lake Geneva by calling for immediate access to humanitarian aid for 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, urging both sides to reach a comprehensive settlement based on the U.N. Geneva Communique, under which world powers called in 2012 for a transitional government to oversee change in Syria.
"Great challenges lie ahead but they are not insurmountable," he added.
Western powers and Russia have sought to set aside their own sharp differences over whether Assad must be forced to make way for an interim administration and have backed the conference as a way to stop the spread of communal and sectarian violence spreading across the region.
Moscow and Washington differ, however, over whether the 2012 accord - known as Geneva 1 - means that Assad must step down immediately. Western powers say that it does.
The new conference, known as Geneva 2, 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.
OPPOSITION, GOVT CLASH
Showing the opposition's determination to see through the demands of the rebels, 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 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.
Russian Foreign Minister Sergei Lavrov repeated Moscow's opposition to "outside players" meddling. But he also said Iran - Assad's main foreign backer - should have a say as world powers tried to prevent the bloodshed spilling across borders.
U.S. Secretary of State John Kerry insisted: "We see only one option, negotiating a transition government born by mutual consent. 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."
Iran was not represented. A last-minute invitation from Ban to attend was revoked on Monday after the Syrian opposition threatened to boycott the talks.
Iranian President Hasan Rouhani said that 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.
As speeches began 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 on the eve of the talks of thousands of photographs apparently showing prisoners tortured and killed by the government was cited by Jarba and Western ministers. The president, who succeeded his father 14 years ago, insists he can win re-election and can defeat "terrorism."
Assad has been protected by Russia, his main arms supplier, which dislikes Western attempts to overthrow incumbent leaders.
But Washington and Moscow share alarm at the spread of the violence that has already killed more than 130,000 Syrians. Having set aside their differences last year to co-sponsor the talks that are finally getting under way, Russia and the United States profess an urgent common goal of halting the bloodshed.
"It is hard to have expectations at the back of all this," said a source at the talks who has advised the opposition. "But Moscow and Washington are genuine on ending the conflict. They are sincere and this meeting is not for show."
ROOTS OF WAR
Neither side in Syria has either appeared able to complete a victory. Though much divides the rebels, who have been fighting among themselves, they are united in wanting Assad out. So reaching a settlement that satisfies both sides seems a distant prospect.
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. Iran and its Lebanese Shi'ite ally Hezbollah have helped Assad. And violence has spread, notably to neighbouring 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."
(Additional reporting by Tom Miles, Gabriela Baczynska, Dominic Evans, Samia Nakhoul, Khaled Yacoub Oweis, Lesley Wroughton and Mariam Karouny in Montreux Guy Faulconbridge in London and Laila Bassam, Paris Hafezi in Ankara, Alexander Dziadosz and Stephen Kalin in Beirut; Writing by Alastair Macdonald; editing by Janet McBride)