MONTREUX, Switzerland - The biggest push yet to end Syria’s bloodshed was marked by fiery exchanges Wednesday as the warring sides and global powers clashed over President Bashar al-Assad’s fate at a UN peace conference.

UN leader Ban Ki-moon opened the discussions in Switzerland with a plea for differences to be set aside, but there was little sign of compromise. Branding the opposition “traitors” and foreign agents, Syrian officials insisted Assad will not give up power, while the opposition said he must step down and face trial.

“After nearly three painful years of conflict and suffering in Syria, today is a day of hope,” Ban said. “You have an enormous opportunity and responsiblity to render a service to the people of Syria.” Meeting for the first time since the start of the conflict in March 2011, the two sides could not be further apart at the “Geneva II” conference in Montreux on the shores of Lake Geneva.

The opposition arrived with a sole aim - toppling Assad - while the regime says any talk of removing the Syrian leader is a “red line” it will not cross. “Assad will not go,” Syrian Information Minister Omran al-Zohbi said on the sidelines of the conference, accusing supporters of the opposition of backing radical militants. “If you want to support Al-Qaeda, go ahead,” Zohbi said.

Syrian Foreign Minister Walid Muallem wasted no time firing a broadside at the opposition in his opening speech, which went on long beyond the allotted time of less than 10 minutes, forcing Ban to repeatedly ask him to wrap it up.

“They (the opposition) claim to represent the Syrian people. If you want to speak in the name of the Syrian people, you should not be traitors to the Syrian people, agents in the pay of enemies of the Syrian people,” Muallem said.

Ahmad Jarba, the head of the opposition Syrian National Coalition, called on the regime to “immediately” sign a deal reached at the last peace conference in Geneva in 2012 setting out “the transfer of powers from Assad, including for the army and security, to a transition government.” He said that would be “the preamble to Bashar al-Assad’s resignation and his trial alongside all the criminals of his regime.” Syrian state television broadcast Jarba’s speech in a split screen alongside footage of death and destruction under the heading “Terrorist Crimes in Syria”.

Leading a series of sharp US accusations against the Syrian regime, Secretary of State John Kerry insisted Assad cannot be part of any transitional government.

“There is no way, not possible in the imagination, that the man who has led the brutal response to his own people could regain legitimacy to govern,” Kerry said.

Russian Foreign Minister Sergei Lavrov warned the talks will “not be simple, they will not be quick” but urged both sides to seize a “historic opportunity”.

About 40 nations and international groups were gathered, but no direct talks are expected until Friday - when opposition and regime delegations will meet in Geneva for negotiations that officials have said could last seven to 10 days.

Meanwhile, Syria on Wednesday rejected a report purporting to show the systematic torture and killing of about 11,000 detainees, calling it an attempt to undermine peace efforts as diplomats gathered in Switzerland for talks.

The 31-page report released on Monday contains 55,000 images of emaciated and mutilated corpses. Bearing signs of torture, some of the corpses had no eyes. Others showed signs of strangulation or electrocution.

The images were supplied by a Syrian military defector who had worked as a military police photographer for 13 years. His identity has not been released to the media.

Former war crimes prosecutors who authored the report said Syrian officials could face war crimes charges as a result of the evidence, which they said evoked images of Nazi death camps. But Syria’s Ministry of Justice said the report was “politicised” and “lacking objectivity and professionalism”.

“Releasing the report one day before the ‘Geneva 2’ conference provides categorical evidence that its goal ... is to undermine efforts aimed at achieving peace in Syria,” the ministry said in a statement carried on Syrian state television.