Leader of the Syrian opposition Moaz al-Khatib, who threatened to boycott talks with US Secretary of State John Kerry, heads a deeply divided and largely ineffective political opposition.
He urged President Bashar al-Assad to go, he said: "Behave for once in your life as a human being, make one reasonable decision in your lifetime."
As Mr Kerry and Mr al-Khatib leave the open meeting, a protester raises a sign in front of them saying, "USA, EU, Saudi support terrorists."
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Top temperature 14 Celsius (57 F).
The Prime Minister will say Moscow's brazen flouting of international law represents a threat to the basis of democracy across Europe.