More than 600 people have taken part in protests in Bristol city centre and in Bedford Square, Exeter against UK involvement in air strikes in Syria.
The demonstrations were a call to "halt the military intervention and demand MPs and parliament represent this opposition".
The marches were just two of many planned across the country.
They were organised by the Stop the War Coalition, who call the military action a "potentially catastrophic move" from the Government and allies.
RAF Tornados joined US and French forces in targeting sites across the country on Friday night in response to an alleged chemical weapons attack in the Syrian city of Douma.
The Prime Minister told MPs on Monday that the decision to launch airstrikes against Syria without a vote in Parliament "not just morally right but also legally right."
She also hit back at suggestions from Labour leader Jeremy Corbyn that the UK was following orders from the the US by joining action against the Syrian regime.
Mrs May told parliament: "Let me be absolutely clear, we have acted because it is in our national interest to do so.
"It is in our national interest to prevent the further use of chemical weapons in Syria and to uphold and defend the global consensus that these weapons should not be used.
"For we cannot allow the use of chemical weapons to become normalised within Syria, on the streets of the UK or elsewhere.
"So we have not done this because President Trump asked us to do so. We have done it because we believed it was the right thing to do and we are not alone."