* Wednesday conference buffeted by events, low expectations
* Withdrawn U.N. invite to Iran still rankles
* Lawyers say photos may show government killing thousands
* Suicide bomber kills four in Beirut Hezbollah stronghold
Syrian and international delegates were arriving in Switzerland on Tuesday on the eve of peace talks that few believe can succeed as the three-year-old civil war and geopolitical acrimony it has brought show no sign of abating.
Opponents of President Bashar al-Assad, pressured to attend Wednesday's first direct negotiations by their Western backers, cited new, photographic evidence of widespread torture and killing by Syria's government in renewing their demand that Assad must quit and face an international war crimes trial.
War crimes lawyers said a vast, smuggled cache of images from a Syrian military police photographer gave clear evidence of systematic abuse and murder of some 11,000 detainees. One of three former international war crimes prosecutors who signed the report compared the images from Syria to the "industrial-scale killing" of Nazi death camps.
The delegation from Damascus, led by Assad's foreign minister, was briefly held up at Athens due to an argument over whether EU trade sanctions permitted refuelling the plane. Assad has insisted he may be re-elected and says the talks should focus on fighting "terrorism" - his term for his enemies.
The United Nations, along with co-sponsors Russia and the United States, may at least be relieved if and when the two sides sit down at the Montreux Palace hotel on Lake Geneva. A day of diplomatic chaos on Monday had threatened to scupper the event entirely, after U.N. Secretary-General Ban Ki-moon gave a last-minute invite to Iran, Assad's main foreign supporter.
The invitation was withdrawn after a boycott threat from the opposition, Western pressure and Iran's insistence it had never agreed to the condition Ban set for attendance - that it endorse a previous peace conference, at Geneva in 2012, which called for Assad to make way for a transitional administration.
Narrowing the gap between the warring parties seems a tall order and diplomats at the United Nations stress the meeting at Montreux on Wednesday, to be followed possibly by further talks in Geneva from Friday, is only a beginning. It could produce some deals to ease civilian suffering and exchange prisoners.
Not only are both sides still committed to a fight on the frontlines, where victory continues to elude either party, but most of the myriad rebel groups have disowned the National Coalition opposition group for agreeing to talk.
And while the United Nations mediator, Lakhdar Brahimi, notionally has the consensus support of world powers, the uproar over the invitation to Tehran illustrated how the war has divided Western powers from Russia and set the Sunni Arab states which back the rebels against Shi'ite Iran.
The spread of violence, which has already killed more than 130,000 and driven a third of Syria's 22 million people from their homes, has, however, given a new, common impetus to international efforts to end the bloodshed.
In Beirut on Tuesday, a suicide bomber killed four people in a stronghold of Hezbollah, the Iranian-backed Lebanese Shi'ite militia. It has sent fighters to help Assad, a member of Syria's Alawite minority, an offshoot of Shi'ite Islam.
At a camp in Lebanon, a Syrian named Abu Shadi said the refugees just wanted a deal to let them return to their towns:
"Hopefully they will find us a solution in this conference so we can go back home," he said. "Find us a solution so that we can go back home. We're really tired of this."
It has been 18 months since the previous international peace conference, dubbed Geneva-1, ended in failure, and all other diplomatic initiatives have also proven fruitless.
"At best, Geneva 2 will reconfirm agreements made during the first Geneva conference, call for ceasefires, maybe prisoners swap and so on," said one Western diplomat.
"At the same time, those taking part in the talks are de facto giving legitimisation to Damascus. They are talking to Assad's government on the other side of the table.
"And so the show would go on while Assad stays in power."
Speaking on his arrival in Switzerland, Badr Jamous, secretary-general of the opposition National Coalition and member of its negotiating team, told Reuters: "We will not accept less than the removal of the criminal Bashar al-Assad and changing the regime and holding the murderers accountable."
The talks could increase the already ferocious internal strife among rival opposition factions, however. The conference is being boycotted by the powerful Sunni Islamist factions that control substantial territory inside Syria. They have denounced the exiled political opposition as traitors for attending.
The main ethnic Kurdish faction, which controls a large swathe of the northwest, has not been invited.
The U.N. secretary-general arrived in Geneva, having nearly torpedoed them with his botched invitation to Iran. Aides shielded him from reporters' questions about the affair.
Western countries have long insisted Tehran sign up to the final statement from Geneva-1 before it could attend other talks. Ban said Iran's foreign minister had told him Tehran accepted the 2012 statement, which includes a requirement that Syria set up a transitional government.
But Tehran said it had agreed to no such thing.
A Western diplomat described the day as "a real mess" and said Ban had made a gaffe that had almost led to the entire conference being cancelled and replaced with a bilateral meeting between Russia and the United States.
Iranian Deputy Foreign Minister Abbas Araqchi blamed Washington for the confusion. "We had repeatedly said that Iran would not accept any precondition," he told state television.
"We were willing to participate at the Geneva 2 conference, but because of America's illogical persistence on imposing preconditions on Iran, we will not take part."
EMACIATED AND ABUSED
The bleak consequences of the war were illustrated starkly in photographs of the emaciated and abused bodies of detainees, released in a report commissioned by London law firm Carter-Ruck, hired by Qatar, a supporter of Assad's foes.
The report, by three senior lawyers who have worked for international war crimes tribunals and three forensic experts, said they believed the pictures and the photographer's account were credible evidence Assad's government had systematically tortured and killed as many as 11,000 detainees.
They said they had been shown a cache of 55,000 images, most of which were provided by a source who identified himself as a Syrian police photographer whose job included documenting deaths in Assad's jails on behalf of the authorities.
The man, who sometimes had to photograph 50 bodies a day, had defected with digital copies of the images, and the lawyers believed he was a credible source.
"The bodies he photographed since the civil war began showed signs of starvation, brutal beatings, strangulation and other forms of torture and killing," the lawyers wrote. "In some cases the bodies had no eyes."
One of the authors, Desmond de Silva, former chief prosecutor of a war crimes tribunal for Sierra Leone and one of Britain's top lawyers, said the evidence documented "industrial-scale killing" reminiscent of Nazi death camps.
"Some of the images we saw were absolutely reminiscent of pictures of people who came out of Belsen and Auschwitz," de Silva said. "It is the tip of the iceberg because this is 11,000 in just one area."
"This is not to say that the people on the other side have been free of serious crime. I think there is evidence that has led very responsible people to say there have been crimes committed on both sides," he said.
"But this industrial killing of people in detention in our view is clearly that of the government."