players" meddling in Syria's affairs but he also said Iran - Assad's main foreign backer - should have a say.
Kerry, in his brief speech to the floor, said negotiations would be "tough and complicated" but insisted there was "no way" Assad could stay on with a transitional government.
"One man can no longer hold an entire nation or region hostage," Kerry said, while also adding there would be no room in government for "terrorists" among the rebel forces.
Assad backer Iran was not present. A last-minute invitation from Ban to attend was revoked on Monday after the Syrian opposition threatened to boycott the talks, since Iran shares Assad's view that he should not lose power.
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.
French Foreign Minister Laurent Fabius said on arrival: "The real conference begins in two days. If it fails it will be a real crisis."
Suhair Attassi, a member of the opposition National Coalition, said in Montreux that Moualem's refusal to accept Assad should go meant it would be up to his Russian ally to press him to accept the international demand for a transition.
"Now there is the responsibility on Russia to put pressure on Assad," he said. "The transition for us