The light-hearted moment came in response to a joke by the House of Commons Speaker and was not related to the conflict, but many viewers watching the culmination of the 11-hour debate missed the transition.
“Not only have innocent people been condemned to die, but the MPs are laughing about it,” one person wrote, while another said they were “sickened”.
John Bercow read out the result of the historic vote in a solemn silence to MPs packed in the House of Commons chamber.
Politicians lined benches and stood with their hands clasped as the Speaker announced victory for the Government by 397 votes to 223.
“It frustrates me so much, how do they sleep knowing innocent civilians are going to die,” one person asked.
But many viewers defended MPs and pointed out that the laughter was not directly related to the vote on Syria.
“The Speaker was answering a point of order on a different subject. Don’t let that spoil your indignant rant,” Barry White wrote.
“I guess we don't know what it's like to debate for 12 hours with the tensions involved and then reach an end,” said another Twitter user.
It was not the only moment of levity during the marathon debate. Mr Bercow was also congratulated for spending more than seven hours in his chair by Labour MP Barry Gardiner, who added: “I must admire your bladder”.
In pictures: Syria air strikes (2014)
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Mr Bercow replied: “I'm extremely grateful.”
Tom Tugendhat, the Tory MP for Tonbridge and Malling, added: “May I also give praise to your endurance rather than any other part of your anatomy.”
David Cameron has said MPs took the “right decision to keep the UK safe” after Wednesday night’s vote.
RAF jets and drones were poised to carry out air strikes on the jihadist terror group within hours of the decision.
Additional reporting by PA
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