More than 100 missiles were fired on the Syrian regime in three separate blitzes aimed at chemical weapon sites in Damascus and the Homs area.
Theresa May is in Parliament today giving a statement to MPs about the reasons behind her decision to join the airstrikes.
Labour’s Jeremy Corbyn has questioned the validity of the action, while the Scottish National Party says it wants to force a vote on the issue.
Where are the UK protests happening? What time?
Three events are going ahead this afternoon in London, Bristol and Exeter organised by the campaigning group Stop the War coalition.
In Bristol, a Trump & May – No More Bombs on Syria! event starts at 5pm by the Water Fountains in Bristol City.
Protestors will also gather in Parliament Square, London at 5.30pm, outside Westminster where politicians are currently engaged in a war debate.
A third event called Don’t Bomb Syria starts at 5.30pm in Bedford Square Exeter.
Other similar events are being held worldwide including in the US and Iraq.
In a statement on its website, Stop the War Coalition said it “strongly condemned” the missile blitz adding it will have “only brought the Syrians “more misery and destruction”.
It said: “They will have done nothing to end the war or alleviate their suffering.
“In sanctioning killing at the behest of Donald Trump, Theresa May deliberately avoided consulting parliament and risked dramatically widening the war.
“The overwhelming majority of people in this country oppose this action just as they have opposed the series of wars of the last seventeen years.
“Following protests around the country yesterday the Stop the War Coalition is calling for a further protest in Parliament Square this Monday 16 April the day Parliament reconvenes.”
Theresa May has defended her actions saying the government has the right to act without consulting parliament and there was “clear evidence” to intervene.
Addressing Parliament, she added the UK had “explored every diplomatic channel” but said military intervention had proven necessary and the UK was acting in the national interest, not just following US President Trump.
Jeremy Corbyn challenged this claim saying a diplomatic process led by John Kerry and Sergei Lavrov after a chemical weapons attack in 2013 resulted in the destruction of 600 tonnes of chemical weapons.
OPCW inspectors are currently on a fact-finding mission to assert if chemical weapons were used against the Syrian people in an attack last month.
Theresa May said the UK government “strongly supports” the mission.