Supporters of President Bashar al-Assad pelted the U.S. ambassador to Syria with tomatoes and eggs on Thursday, an opposition source said.
The incident took place in central Damascus, as Ambassador Robert Ford was nearing the office of the opposition National Coordination Committee for Democratic Change for talks with its chairman Hassan Abdel Azim.
Local reports said protesters surrounded the ambassador's car and threw eggs and tomatoes at it. After police dispersed the mob, Ford held the scheduled meeting.
The Syrian government has used force to crush opposition protests, which followed a wave of uprisings in other Arab countries. The UN says over 2,000 people have died since the start of the protests in March.
The United States and the EU have already imposed unilateral sanctions on Syria and have called for Assad to step down.
On September 21, U.S. President Barack Obama urged the UN Security Council to impose new sanctions against Syria over its crackdown on opposition protesters. On September 23 the EU imposed a fresh round of sanctions.