Syrian president Bashar al-Assad should quit so the country can undergo political reform, shadow foreign secretary Douglas Alexander said today.
Mr Alexander told delegates at Labour's annual conference in Liverpool the time had come for the Syrian leader to step down, as the death toll mounts amid bloodshed from the popular revolt.
The shadow foreign secretary said: "Let us say today from this conference to President Assad, 'We cannot and will not accept your violence against your own people, we will use every diplomatic measure to stop it. You must go, and go now."'
The demand came as Mr Alexander said the uprisings sweeping the Middle East and North Africa had seen "the downfall of old autocrats and old assumptions".
He said the hope shown by demonstrators had changed history and the UK's attitudes to the region.
He admitted: "Too often in the past, the West has backed stability over democracy in the Middle East.
"So I'm so proud that this year, this party chose to stand with these young people, and against the old autocrats.
"That choice meant I could stand on the street in Tunis a few months ago and look them in the eye."
Mr Alexander praised the British Government's decision to intervene in Libya as dictator Colonel Muammar Gaddafi prepared to attack the country's second city, Benghazi.
Mr Alexander urged activists at the Arena and Convention Centre to accept Britain should adopt "a foreign policy that is realistic about what we can achieve alone, but idealistic about what we can achieve together".
He was "optimistic about our country's enduring strengths, despite the riots, despite the cuts and the deficit and despite the flat-lining economy".
And rallying the party faithful, he added: "Britain today is so much better than its Government."