Demonstrators waved the two-star flag of the Syrian government and pictures of President Bashar al-Assad outside the UN's office in the Mazzeh neighbourhood of the Syrian capital.
Their placards read "Iraq will not happen again, this is Assad's Syria," and "Trump supports terrorism."
Some protesters chanted, "Death to America, death to Israel!"
US warships on Friday fired 59 Tomahawk missiles at a central Syrian air base from which Washington believes government jets launched a chemical attack that killed at least 87 civilians.
"We came to denounce the American strike," said Ayyad Talab, head of the university students' branch in the National Syrian Students Union (NSSU), which organised the protest.
"We want to say that we are ready to defend our country, armed with our work, our minds, and our ideas and with our souls if necessary," he told AFP.
The NSSU submitted a letter, addressed to UN Secretary General Antonio Guterres, that the United Nations mission in Damascus said it would forward to his office.
The US retaliatory strike marked the first time the United States has intervened directly in the Syrian war against Assad's government.
Syria's conflict erupted in March 2011 with anti- government protests but has since evolved into a complex war among government forces, jihadists, rebels, and Kurds.
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