Police on horseback intervened to separate the two groups who for several minutes appeared on the verge of coming to blows.
"The situation in Syria is very complicated, and dropping bombs on that situation is not going to solve anything," said Lacy MacAuley wearing a "Hands off Syria" headband.
"It's not going to make the Assad regime stop, it's not going to make the Free Syrian Army stop," added the 34-year-old.
"It's not our job," he added. "They're their own country, they're a sovereign nation and they don't want us. Plus both sides are terrible, we have no need to get in there."
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For Yahya Abo, a 26-year-old dual Syrian and US citizen, a strike on Syria would lead to "a big war, like third World War" that would destabilise the Middle East.
The roughly 50 backers of the Syrian opposition meanwhile shouted slogans against President Bashar al-Assad, with one holding up a sign that compared him to Adolf Hitler.