Syria's government and main political opposition have traded bitter accusations as a major peace conference begins in Switzerland.
UN Secretary General Ban Ki-moon urged delegates to engage in constructive discussions, but neither side appeared prepared to abandon their positions.
Syria's foreign minister and the National Coalition
president delivered angry statements
This first day of talks was always expected to be
somewhat ceremonial, with one formal speech after
another. But already predictable fault lines are emerging.
Syria's Foreign Minister Walid Muallem said his country
was engaged in a war against terrorist groups, adding
that only the Syrian people could decide on President
Assad's future. The opposition leader Ahmad Jarba said
the human rights violations in Syria were reminiscent of
Nazi Germany, and suggested President Assad's
departure was a precondition for peace.
These two men won't be in the negotiating room together
until Friday, then the talking will be in private, but it's
expected to be equally tough.
Key figures here - including Ban Ki-moon, John Kerry,
Sergei Lavrov and William Hague - have publicly
reminded the warring parties of the real price of this
conflict. An "all encompassing disaster" said the UN
secretary general, which Russia's foreign minister added
had caused "incalculable suffering" to the Syrian people.
US Secretary of State John Kerry said there was "no way possible" President Bashar al-Assad could remain in power.
The conflict has left more than 100,000 dead and millions displaced.
The summit is discussing the Geneva communique which lays out a political transition plan for Syria. But the key issue is President Assad's future.
It will hear from about 40 foreign ministers on Wednesday before direct Syrian talks are scheduled to begin in Geneva on Friday.
This would be the first face-to-face meeting between the Syrian government and the main opposition - the National Coalition - since the conflict began in 2011.
The BBC's Paul Wood, in Montreux, says there were some extraordinarily ill-tempered scenes and some very direct language as the conference got under way.
Syria's Foreign Minister Walid Muallem said some states attending the talks had "Syrian blood on their hands" and were trying to destabilise the country.
Addressing US Secretary of State John Kerry, he said: "No-one in the world has the right to confer or withdraw the legitimacy of a president, a constitution or a law, except for the Syrians themselves."
Mr Muallem ran far over the allotted 10-minute slot for each speaker, ignoring Mr Ban's attempts to intervene.
"You live in New York. I live in Syria," Mr Muallem told Mr Ban. "I have the right to give the Syrian version here. After three years of suffering, this is my right."
The US state department condemned Mr Muallem's remarks as "inflammatory rhetoric", and urged the government to take "real, concrete steps to increase humanitarian access and improve the lives of the people suffering the most".
The head of the National Coalition, Ahmad Jarba, said in his speech it had not been the opposition's choice to take up arms, but "was the choice imposed by the Syrian regime".
He displayed a photograph taken from a report by three war crimes investigators which alleged "systematic" torture and execution of opposition detainees in Syria. The report was released on Tuesday but dismissed as not credible by Damascus.
A UN-backed meeting in 2012 issued the document and
urged Syria to:
Mr Jarba called on the government to immediately sign the Geneva document and transfer power to a transitional authority.
"For the Syrians, time is now blood."
Our correspondent says that when the talks go behind closed doors there will perhaps be a more constructive tone - with discussion of practical matters such as ceasefires and access for humanitarian aid.
In his opening speech, Mr Ban urged all parties to engage "seriously and constructively".
"Let me not mince words - the challenges before you and before all of us are formidable. But your presence here raises hope," he said.
Russian Foreign Minister Sergei Lavrov said the talks "will not be simple, will not be quick", but that there was "a historic responsibility on the shoulders of all participants".
He also repeated his insistence that Iran should be involved.
The UN withdrew its invitation to Iran this week over its refusal to back the Geneva communique.
Iran's President Hasan Rouhani said on Wednesday that the "lack of influential players" attending meant he doubted "its ability to resolve the Syria crisis".
Mr Kerry used his opening remarks in Montreux to remind the conference the uprising had begun as a peaceful process, but said the government had responded "with ever-increasing force".
"We see only one option: negotiating a transition government born by mutual consent," he said. "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."
UK Foreign Secretary William Hague said "thousands more innocent Syrians will pay the price" if the talks fail. The government, he said, "bears a particular responsibility for this crisis and can do the most to end it".
International delegates to Geneva II have played down hopes of a breakthrough, saying the talks should be seen as the first step in a process.