Opposition leader Ahmed Jarba accused the president of Nazi-style war crimes following the publicaton of pictures showing industrial scale torture. He demanded the Syrian government delegation at the one-day meeting in Switzerland immediately sign up to an international plan for handing over power.
However, Syrian Foreign Minister Walid al Moualem insisted Mr 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.
America and Russia, co-sponsors of the conference, which UN officials hope will lead to negotiations in Geneva tomorrow, also revealed their differences over Mr Assad during a day of formal presentations at Montreux, beside Lake Geneva.
The talks reflect mounting global concern that a war that has killed more than 130,000 and left millions homeless is spilling beyond Syria and fuelling sectarian militancy abroad, But there was little sign any party was ready to make concessions.
American Secretary Of State John Kerry echoed the rebel view that there is "no way" Mr Assad can stay under the terms of a 2012 international accord calling for an interim coalition. Russian Foreign Minister Sergei Lavrov said all sides must have a role and criticised "one-sided interpretations" of that 2012 pact.
Saudi Arabia, which backs the Sunni rebels, called for Iran and its Shi'ite Lebanese ally Hezbollah to withdraw forces from Syria. Iran, locked in a sectarian confrontation across the region, was absent, shunned by the opposition and the West for rejecting calls for a transitional government. Its president said Tehran's exclusion meant talks were unlikely to succeed.
The conference 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 Mr Assad's delegates.
UN Secretary-General Ban Ki-moon opened proceedings at Montreux by calling for immediate access for humanitarian aid convoys to areas under siege.
He said: "After nearly three painful years of conflict and suffering in Syria, today is a day of fragile but real hope."
He urged both sides in Syria to reach a full settlement based on a 2012 UN Geneva Communique, under which world powers called for a transitional government.
Many of the more than 40 government representatives echoed concerns about the human cost of the war and the dangers of escalation posed by heavily armed international militants.
But there was little sign of compromise on the central issue of whether Mr Assad, who inherited power from his father 14 years ago, should make way for a government of national unity.
He himself says he could win re-election this year and his fate has divided Moscow and Washington. Both endorse the conclusions of the 2012 meeting of world powers, known as Geneva 1, but differ on whether it means Mr Assad must go now.
Opposition leader Mr Jarba called for the government delegation to turn against their president before negotiations start. 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."
Turning around the government's accusations that the rebels have fostered al Qaeda and other militants, Mr Jarba said it was Assad's forces which, by targeting mainstream opposition groups, had created the conditions for al Qaeda to thrive.
Mr Moualem exchanged sharp words with Mr Ban as he spoke well beyond a 10-minute limit the head of the UN had requested. The Syrian foreign minister called on foreign powers to stop "supporting terrorism" and to lift sanctions.